iris: Support more RGBX pipe formats.

iris: Support more RGBX pipe formats.

Without them, the state tracker falls back to an RGBA format, but it
doesn't always manage to override the swizzle for us.  So we lose the
information that the API expects an X channel, where alpha is garbage
and reads back as 1.  We have no equivalent ISL RGBX format for these,
so we just use RGBA directly and override the swizzle in all cases.