drm/simple_kms_helper: 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.

I don't see any actual users of drm_simple_kms_helper yet, so
no actual plane clipping bugs to fix.

Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1469549224-1860-10-git-send-email-ville.syrjala@linux.intel.com