intel: Remove intel_context::has_xrgb_textures/has_luminance_srgb.

intel: Remove intel_context::has_xrgb_textures/has_luminance_srgb.

Seeing as they were only used once (in the same function they were
defined), having them as context members seemed rather pointless.

Remove them entirely (rather than using local variables) since the
chipset generation checks are actually just as straightforward.

While we're at it, clean up the remainder of the if-tree that set them.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>