drm/i915: enable HiZ Raw Stall Optimization

Enable HiZ optimization for Haswell.

Thought we had already done a pull request...

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>