drm/i915: enable HiZ Raw Stall Optimization

backported to SteamOS kernel branch.

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

Signed-off-by: Chia-I Wu <olv at lunarg.com>
Cc: Ian Romanick <ian.d.romanick at intel.com>
Cc: Chad Versace <chad.versace at linux.intel.com>
Merge pull request #4 from LunarG/enable-HiZ-on-Haswell

drm/i915: enable HiZ Raw Stall Optimization