st/mesa: Fix GL_MAP_COLOR with glDrawPixels GL_COLOR_INDEX

Documentation for glDrawPixels with GL_COLOR_INDEX says:
 "If the GL is in color index mode, and if GL_MAP_COLOR is true,
  the index is replaced with the value that it references in
  lookup table GL_PIXEL_MAP_I_TO_I"

We are always in RGBA mode and there is nothing in documentation
about GL_MAP_COLOR in RGBA mode for GL_COLOR_INDEX.

Scale and bias are also only applicable for RGBA format and not
mentioned for GL_COLOR_INDEX.

Thus the behaviour will be on par with i965.

Fixes: gl-1.0-drawpixels-color-index
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
15 jobs for !567 with fix/gallium-gl-map-color in 17 minutes and 43 seconds (queued for 1 second)
latest detached
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passed #213148
ubuntu

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  Build+Test
passed #213149
autotools

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passed #213151
meson-clang

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passed #213157
meson-gallium-clover-llvm

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passed #213158
meson-gallium-clover-llvm39

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passed #213156
meson-gallium-drivers-other

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passed #213155
meson-gallium-radeonsi

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passed #213159
meson-gallium-st-other

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passed #213150
meson-gallium-swr

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passed #213154
meson-glvnd

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passed #213153
meson-loader-classic-dri

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passed #213152
meson-vulkan

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passed #213161
scons-llvm

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passed #213160
scons-nollvm

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passed #213162
scons-swr

00:06:38