drm/imx: fold ipu_plane_disable into ipu_disable_plane

ipu_disable_plane is the only left caller of ipu_plane_disable.
Having those 2 similar named functions is confusing and superfluous,
so fold them into 1.

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