intel/anv: Fix condition for planar yuv surface

Test the sampler->conversion for NULL pointer before dereferencing it.

Fixes: Regressions in VulkanCTS.
Fixes: 22631611 "intel/anv: Fix condition to set MipModeFilter for YUV surface"
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
34 jobs for !9111 with planar-yuv in 5 minutes and 59 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Sanity
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sanity

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  Container
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aarch64 manual
arm_build

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arm_test-base

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windows shell 1809 mesa manual
windows_build_vs2019

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android_build

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arm64_test

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i386_build

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kernel+rootfs_amd64

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ppc64el_build

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s390x_build

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x86_build

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x86_test-gl

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meson-clang

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meson-clover

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meson-release

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meson-testing

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meson-testing-asan

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meson-vulkan

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  Build Misc
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meson-android

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aarch64
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aarch64
meson-arm64-build-test

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aarch64
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meson-i386

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meson-mingw32-x86_64

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