i965: Support accelerated blit for depth 30 formats. (v2)

i965: Support accelerated blit for depth 30 formats. (v2)

Extend intel_miptree_blit() to handle at least
ARGB2101010 -> XRGB2101010, ARGB2101010 -> ARGB2101010,
and XRGB2101010 -> XRGB2101010 via the BLT engine,
but not XRGB2101010 -> ARGB2101010 yet.

This works as tested under Compiz, KDE-5, Gnome-Shell.

v2: Restrict BLT fast path to exclude XRGB2101010 -> ARGB2101010,
    as intel_miptree_set_alpha_to_one() isn't ready to set 2 bit
    alpha channels to 1.0 yet. However, couldn't find a test case
    where this specific blit would be needed, so maybe not much
    of a point to improve here.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>