[Mesa-dev] Enable HiZ on Haswell

[Kenneth Graunke]

(I'm sending this out for Chad, whose email doesn't appear to be working.)

Enable hiz by setting intel_context::has_hiz.  However, to work around
a hardware bug, we selectively enable hiz for only nicely aligned miptree
slices.

No Piglit regressions on Haswell 0x0d26 rev07 when based atop
mesa-master-97e40a5.

Improves the performance of GLB27_TRex_C24Z16_FixedTimeStep by 18.52%
(hsw-0x0d26-rev07; kernel-3.9.0-rc1; GLBenchmark 2.7.0 Release a68901;
samples=3).