i965/blorp: allow multisample blorp clears

i965/blorp: 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, we can do multisample blorp clears.

However, we need to do a few other pieces of bookkeeping to make them
work correctly in all circumstances:

- Since blorp clears may now operate on multisampled window system
  framebuffers, they need to call
  intel_renderbuffer_set_needs_downsample() to ensure that a
  downsample happens before buffer swap (or glReadPixels()).

- When clearing a layered multisample buffer attachment using UMS or
  CMS layout, we need to advance layer by multiples of num_samples
  (since each logical layer is associated with num_samples physical
  layers).

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

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>