i965: Use MESA_FORMAT_B8G8R8X8_SRGB for RGB visuals

i965: Use MESA_FORMAT_B8G8R8X8_SRGB for RGB visuals

Previously if the visual didn't have an alpha channel then it would
pick a format that is not sRGB-capable. I don't think there's any
reason not to always have an sRGB-capable visual. Since 28090b30 there
are now visuals advertised without an alpha channel which means that
games that don't request alpha bits in the config would end up without
an sRGB-capable visual. This was breaking supertuxkart which assumes
the winsys buffer is always sRGB-capable.

The previous code always used an RGBA format if the visual config
itself was marked as sRGB-capable regardless of whether the visual has
alpha bits. I think we don't actually advertise any sRGB-capable
visuals (but we just use sRGB formats anyway) so it shouldn't make any
difference. However this patch also changes it to use RGBX if an
sRGB-capable visual is requested without alpha bits for consistency.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92759
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Suggested-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>