iris: Convert RGBX to RGBA always.

iris: Convert RGBX to RGBA always.

In i965, we disable the use of RGBX formats, so the higher layers of
Mesa choose the equivalent RGBA format, and swizzle the alpha channel to
1.0.

However, Gallium won't do that. We need to explicitly convert it to
RGBA.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>