drm/vc4: Use drm_atomic_helper_check_plane_state() to simplify the logic

drm_atomic_helper_check_plane_state() takes care of checking the
scaling capabilities and calculating the clipped X/Y offsets for us.

Rely on this function instead of open-coding the logic.

Incidentally, it seems to fix a problem we had with negative X/Y
positioning of YUV planes.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180803092231.26446-3-boris.brezillon@bootlin.com