nir: improve convert_yuv_to_rgb

Use a different arrangement of constants to allow more ffma.

A vec4 backend will now use 3 fma for yuv_to_rgb. On freedreno/ir3, it is
down from 10 to 7 alu (4 fma, 3 mul, 3 add to 7 fma). Other backends
shouldn't be hurt.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Eric Anholt <eric@anholt.net>
14 jobs for !98 with nir-yuv in 19 minutes and 45 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #257199
debian

00:00:31

 
  Build+Test
passed #257201
meson-clang

00:12:38

passed #257207
meson-gallium-clover-llvm

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passed #257208
meson-gallium-clover-llvm39

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passed #257206
meson-gallium-drivers-other

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passed #257205
meson-gallium-radeonsi

00:05:19

passed #257209
meson-gallium-st-other

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passed #257200
meson-gallium-swr

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passed #257204
meson-glvnd

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passed #257203
meson-loader-classic-dri

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passed #257202
meson-vulkan

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passed #257211
scons-llvm

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passed #257210
scons-nollvm

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passed #257212
scons-swr

00:06:48