mesa: Support flipping three-channel formats

Test system: POWER9 ppc64 (BE) system with a Radeon R5 230.

Before this commit, starting Xorg caused this message:
Assertion failed: !"Invalid array format" (../src/mesa/main/formats.c: _mesa_array_format_flip_channels: 421)

After this commit, Xorg starts successfully.
43 jobs for !3977 with master in 37 minutes and 18 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Container
passed #1754850
aarch64
arm_build

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passed #1754851
aarch64
arm_test

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passed #1754847
x86_build

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passed #1754849
x86_build_old

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passed #1754848
x86_test-gl

00:00:48

 
  Build
passed #1754855
aarch64
meson-arm64

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passed #1754854
aarch64
meson-armhf

00:11:11

passed #1754856
meson-clang

00:09:43

passed #1754859
meson-clover

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passed #1754860
meson-clover-old-llvm

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passed #1754862
meson-i386

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passed #1754853
meson-main

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passed #1754865
meson-mingw32-x86_64

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passed #1754864
gstreamer
meson-ppc64el

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passed #1754863
gstreamer
meson-s390x

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passed #1754852
meson-testing

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passed #1754861
meson-vulkan

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scons

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scons-old-llvm

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passed #1754857
scons-swr

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passed #1754858
scons-win64

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  Test
passed #1754887
db410c
arm64_a306_gles2

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passed #1754884
mesa-cheza
arm64_a630_gles2

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passed #1821718
mesa-cheza
arm64_a630_gles3

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passed #1754885
mesa-cheza
arm64_a630_gles31

00:04:08

passed #1754872
lava-meson-gxl-s905x-libretech-cc
lima-mali450-test:arm64

00:06:08

passed #1754888
llvmpipe-traces

00:00:28

passed #1754868
lava-sun50i-h6-pine-h64
panfrost-t720-gles2:arm64

00:08:50

passed #1754869
lava-rk3288-veyron-jaq
panfrost-t760-gles2:armhf

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passed #1754870
lava-rk3399-gru-kevin
panfrost-t860-gles2:arm64

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passed #1754871
lava-rk3399-gru-kevin
panfrost-t860-gles3:arm64

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passed #1754874
piglit-glslparser

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passed #1754873
piglit-quick_gl

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passed #1754875
piglit-quick_shader

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passed #1754876
test-llvmpipe-gles2

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passed #1754877
test-softpipe-gles2

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passed #1754880
test-softpipe-gles31 1/4

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passed #1754878
test-softpipe-gles3 1/2

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passed #1754881
test-softpipe-gles31 2/4

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passed #1754882
test-softpipe-gles31 3/4

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passed #1754883
test-softpipe-gles31 4/4

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passed #1754879
test-softpipe-gles3 2/2

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failed #1754886
mesa-cheza
arm64_a630_gles3

00:06:20