intel/blorp: Add fast-clear to the special case in MSAA resolves

intel/blorp: Add fast-clear to the special case in MSAA resolves

This doesn't go all the way of avoiding the txf_ms if it's fast-cleared,
however it does at least make us only do it once.  This should improve
performance of MSAA resolves in the presence of lots of clear color.
Without the patch, enabling fast-clears in the multisampling Sascha demo
drops the framerate by about 10%.  With this patch, enabling fast-clears
increases the demo's framerate by 25%.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>