!UPSTREAM: i965: allow multisample blorp clears

Previously, we didn't do multisample blorp clears because we couldn't
figure out how to get them to work.  The reason for this was because
we weren't setting the brw_blorp_params num_samples field consistently
with dst.num_samples.  Now that those two fields have been collapsed
down into one, multisample blorp clears should work just fine.

Note: we still don't do multisample fast color clears; more work needs
to be done to enable those.

TODO:
- Test on gen6.