i965: Support MESA_FORMAT_R8G8B8X8_SRGB.

i965: Support MESA_FORMAT_R8G8B8X8_SRGB.

Valve games use GL_SRGB8 textures.  Instead of supporting that properly,
we fell back to MESA_FORMAT_R8G8B8A8_SRGB (with an alpha channel), which
meant that we had to use texture swizzling to override the alpha to 1.0
when sampling.  This meant shader recompiles on Gen < 7.5 platforms.

By supporting MESA_FORMAT_R8G8B8X8_SRGB, the hardware just returns 1.0
for us, so we can just use SWIZZLE_XYZW, and avoid any recompiles.  All
generations of hardware have supported the format for sampling and
filtering; we can easily support rendering by using the R8G8B8A8_SRGB
format and writing garbage to the X channel.  (We do this already for
the non-SRGB version of this format.)

This removes all remaining shader recompiles in a time demo of "Counter
Strike: Global Offensive" (32 -> 0) on Sandybridge.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87886
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>