i965/msaa: Use MESA_FORMAT_R8 for MCS buffer.

i965/msaa: Use MESA_FORMAT_R8 for MCS buffer.

No functional change.  This patch modifies intel_miptree_alloc_mcs to
allocate the 4x MCS buffer using MESA_FORMAT_R8 instead of
MESA_FORMAT_A8.  In principle it doesn't matter, since we only access
the buffer using MCS-specific hardware mechanisms, so all that's
important is to use a format with the correct size.  However,
MESA_FORMAT_A8 has enough unusual behaviours that it seems prudent to
avoid it.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>