Use drm_plane_mask() where appropriate.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180626194716.12522-6-ville.syrjala@linux.intel.comReviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>