drm/i915/gen9: implement WaEnableSamplerGPGPUPreemptionSupport

WaEnableSamplerGPGPUPreemptionSupport fixes a problem
related to mid thread pre-emption.

Signed-off-by: Tim Gore <tim.gore@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461077152-31899-1-git-send-email-tim.gore@intel.com