i965/gen9: Optimize slice and subslice load balancing behavior.

The default pixel hashing mode settings used for slice and subslice
load balancing are far from optimal under certain conditions (see the
comments below for the gory details).  The top-of-the-line GT4 parts
suffer from a particularly severe performance problem currently due to
a subslice load balancing issue.  Fixing this seems to improve
graphics performance across the board for most of the benchmarks in my
test set, up to ~20% in some cases, e.g. from SKL GT4:

unigine/valley:                3.44% ±0.11%
gfxbench/gl_manhattan31:       3.99% ±0.13%
gputest/pixmark_piano:         7.95% ±0.33%
synmark/OglTexFilterAniso:    15.22% ±0.07%
synmark/OglTexMem128:         22.26% ±0.06%

Lower-end platforms are also affected by some subslice load imbalance
to a lesser degree, especially during CCS resolve and fast clear
operations, which are handled specially here due to rasterization
ocurring in reduced CCS coordinates, which changes the semantics of
the pixel hashing mode settings.

No regressions seen during my tests on some SKL, KBL and BXT
configurations.  Additional benchmark reports welcome on any Gen9
platforms (that includes anything with Skylake, Broxton, Kabylake,
Geminilake, Coffeelake, Whiskey Lake, Comet Lake or Amber Lake in your
renderer string).

P.S.: A similar problem is likely to be present on other non-Gen9
      platforms, especially for CCS resolve and fast clear operations.
      Will follow-up with additional patches fixing the hashing mode
      for those once I have enough performance data to justify it.

Reviewed-by: Kenneth Graunke <[email protected]>

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