i965/blorp: Optimize manual_blend() for compressed multisampled surfaces.

i965/blorp: Optimize manual_blend() for compressed multisampled surfaces.

When downsampling a compressed multisampled surface, we can take a
shortcut to downsample any pixels that were completely covered by a
single primitive.  In this case, the first color value we fetch is the
correct final color for the downsampled pixel, so we can skip the rest
of the blending operation.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>