Adding missing Display mmio reg offset.

Credits-to: Laws, Philip <philip.laws@intel.com>
Cc: He, Shuang <shuang.he@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>