drm/i915: There's no need to mask all 3D_CHICKEN bits on SNB

The need to set all of the mask bits for 3D_CHICKEN3 was required
only for pre-production hardware.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>