i965/blorp: Get rid of redundant num_samples blorp param.

Previously, brw_blorp_params contained two fields for determining
sample count: num_samples (which determined the multisample
configuration of the rendering pipeline) and dst.num_samples (which
determined the multisample configuration of the render target
surface).  This was redundant, since both fields had to be set to the
same value to avoid rendering errors.

This patch eliminates num_samples to avoid future confusion.
!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.

TODO:
- Test on gen6.
- Prevent fast color clear from activating on multisample.
Non-helpful hacks
HACK: try UMS layout