drm/i915: enable HiZ Raw Stall Optimization on HSW

The optimization is available on Ivy Bridge and later, and is disabled by
default.  Enabling it helps certain workloads such as GLBenchmark TRex test.

No piglit regression.

v2
 - no need to save the register before suspend as init_clock_gating can
   correctly program it after resume
 - split IVB change to another commit

Signed-off-by: Chia-I Wu <olv@lunarg.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>