drm/i915: enable HiZ Raw Stall Optimization on IVB

The optimization helps IVB too.  No piglit regression.

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>