Use drm_plane_helper_check_state to clip raw user coordinates to crtc
bounds. This checks for full plane coverage and scaling already, so
we can drop some custom checks. Use the clipped coordinates everywhere.
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>