i965/blorp: Support blits between ARGB and XRGB formats.

i965/blorp: Support blits between ARGB and XRGB formats.

Now that we have support for overriding alpha to 1.0, we can handle
blitting between these formats in either direction.

For now, we only support two XRGB formats: MESA_FORMAT_XRGB8888 and
MESA_FORMAT_RGBX8888_REV.  Most places only appear to worry about the
former, so ignore the latter for now.  We can always add it later.

NOTE: This is a candidate for the 9.1 branch.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Martin Steigerwald <martin@lichtvoll.de>