drm/mediatek: Use drm_plane_helper_check_state()

Replace the use of drm_plane_helper_check_update() with
drm_plane_helper_check_state() since we have a plane state.

This also eliminates the double clipping the driver was doing
in both check and commit phases). And it should fix src coordinate
addr adjustement. Previously the driver was expecting negative dst
coordinates after clipping, which is not going happen, so any clipping
induced addr adjustment simply didn't happen. Neither did the driver
respect any user configured src coordinates, so panning and such would
have been totally broken. It should be all good now.

Cc: CK Hu <ck.hu@mediatek.com>
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Tested-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Acked-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1469549224-1860-9-git-send-email-ville.syrjala@linux.intel.com