tu: Fix xfb when there is a hole at the end

We were handling the case where we had an unassigned output in the
middle of the outputs array, but v->outputs can be smaller than the
shader's info.num_outputs when an output at the end isn't assigned. This
lead to us reading garbage after the end, and assuming that it
corresponded to r0.x and overwriting the xfb entry for some other random
output with the unassigned output's entry.

Part-of: <!12310>
63 jobs for !12310 with review/tu-xfb-fix in 18 minutes and 24 seconds (queued for 11 seconds)
latest detached
Status Name Job ID Coverage
  Sanity
passed sanity #12797789

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  Container
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passed debian/x86_build-base #12797790

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windows shell 1809 mesa

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  Container 2
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passed debian/s390x_build #12797798

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passed debian/x86_build #12797795

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passed debian/x86_test-gl #12797800

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passed debian/x86_test-vk #12797801

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passed kernel+rootfs_amd64 #12797802

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passed kernel+rootfs_arm64 #12797803
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passed kernel+rootfs_armhf #12797804
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  Build X86 64
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passed debian-gallium #12797808

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passed debian-testing-asan #12797806

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passed fedora-release #12797810

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  Build Misc
passed debian-android #12797815

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passed debian-arm64 #12797817
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passed debian-arm64-asan #12797818
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passed debian-arm64-build-test #12797819
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passed debian-armhf #12797816
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passed debian/arm_test #12797814

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  Freedreno
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passed a306-traces #12797827
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passed a530_gles2 #12797828
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passed a530_piglit_shader 2/2 #12797834
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passed a530-traces #12797835
google-freedreno-db820c

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passed a630_egl #12797840
google-freedreno-cheza

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passed a630_gl #12797839
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passed a630_gles2 #12797836
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passed a630_gles3 #12797838
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passed a630_gles31 #12797837
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passed a630_gles_others #12797841
google-freedreno-cheza

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passed a630_piglit #12797845
google-freedreno-cheza

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passed a630-traces #12797846
google-freedreno-cheza

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passed a630-traces-performance #12797848
google-freedreno-cheza manual

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passed a630-traces-restricted #12797847
google-freedreno-cheza

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passed a630_vk 1/2 #12797842
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passed a630_vk 2/2 #12797843
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passed a630_vk_sysmem #12797844
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  Software Renderer
passed softpipe-asan-gles31 #12797851

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passed softpipe-deqp 1/2 #12797849

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passed softpipe-deqp 2/2 #12797850

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