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>

src/mesa/drivers/dri/intel/intel_mipmap_tree.c

@@ -684,7 +684,7 @@ intel_miptree_alloc_mcs(struct intel_context *intel,
       /* 8 bits/pixel are required for MCS data when using 4x MSAA (2 bits for
        * each sample).
        */
-      format = MESA_FORMAT_A8;
+      format = MESA_FORMAT_R8;
       break;
    case 8:
       /* 32 bits/pixel are required for MCS data when using 8x MSAA (3 bits