i965/gen9: Don't allow the RGBX formats for texturing/rendering

i965/gen9: Don't allow the RGBX formats for texturing/rendering

The RGBX surface formats aren't renderable so we internally remap them
to RGBA when rendering. They are retained as RGBX when used as
textures. However since the previous patch fast clears are disabled
for surfaces that use a different format for rendering than for
texturing. To avoid this situation we can just pretend not to support
RGBX formats at all. This will cause the upper layers of mesa to pick
an RGBA format internally instead. This should be safe because we
always override the alpha component to 1.0 for RGBX in the texture
swizzle anyway. We could also do this for all gens except that it's a
bit more difficult when the hardware doesn't support texture
swizzling. Gens using the blorp have further problems because that
doesn't implement this swizzle override.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>