drm/i915/bdw: 3D_CHICKEN3 has write mask bits

The workaround to limit SDE poly depth FIFO to 2 is not applied because
3D Chicken-3 mask bit is not set.

WaLimitSizeOfSDEPolyFifo is only for BDW-A and could be removed.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>