drm/rockchip: 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.

Rockchip looks to handling plane clipping rather well already
(unlikje most arm drm drivers) so there are no function changes
here.

Cc: Yao <mark.yao@rock-chips.com>
Cc: linux-rockchip@lists.infradead.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1469549224-1860-8-git-send-email-ville.syrjala@linux.intel.com