i965/fs: Fix rendering corruption in unigine tropics.

i965/fs: Fix rendering corruption in unigine tropics.

We were allocating registers into the MRF hack region, resulting in
sparkly renering in a few of the scenes.  We could do better
allocation by making an MRF class, having MRFs conflict with the
corresponding GRFs, and tracking the live intervals of the "MRF"s and
setting up the conflicts.  But this is way easier for the moment.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>