drm/i915/bdw: Implement Wa4x4STCOptimizationDisable:bdw

Not implementing this W/A can lead to hangs.

Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Rafael Barbalho <rafael.barbalho@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>