i965/blorp: Use MSDISPMODE_PERSAMPLE rendering when necessary

i965/blorp: Use MSDISPMODE_PERSAMPLE rendering when necessary

This patch modifies the "blorp" WM program so that it can be run in
MSDISPMODE_PERSAMPLE (which means that every single sample of a
multisampled render target is dispatched to the WM program, not just
every pixel).

Previously we were using the ugly hack of configuring multisampled
destination surfaces as single-sampled, and generating sample indices
other than zero by swizzling the pixel coordinates in the WM program.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>