drm/imx: don't wait for vblank and stop calling cleanup_planes in commit_tail

drm_atomic_helper_cleanup_planes only calls the cleanup_fb plane
helpers, which we don't implement as a CMA framebuffer based driver.
There is no reason to wait for vblanks in commit_tail only to do nothing
afterwards.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>