drm/i915: Enable the HiZ RAW Stall Optimization on Broadwell.

This is an important optimization for avoiding read-after-write (RAW)
stalls in the HiZ buffer.  Certain workloads would run very slowly with
HiZ enabled, but run much faster with the "hiz=false" driconf option.
With this patch, they run at full speed even with HiZ.

Improves performance in OglVSInstancing by 3.2x on Broadwell GT3e
(Iris Pro 6200).

Thanks to Jesse Barnes and Ben Widawsky for their help in tracking this
down.  Thanks to Chris Wilson for showing me the new workarounds system.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>