i915: Blit RGBX<->RGBA drawpixels

i915: Blit RGBX<->RGBA drawpixels

The blitter already has code to accommodate filling in the alpha channel
for BGRX destination formats, so expand this to also allow filling the
alpha channgel in RGBX formats.

More importantly for the next patch is moving the test into its own
function for the purpose of exporting the check to the callers.

v2: Fix alpha expansion as spotted by Alexander with the fix suggested by
Kenneth

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Alexander Monakov <amonakov@gmail.com>
Cc: Kristian Høgsberg <krh@bitplanet.net>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit c2d0606827412b710dcaed80268fc665de8c9c5d)