staging: drm/imx: make waiting for idle channel optional

Currently we wait for a channel until it's idle before actually
disabling it. This is not needed for all channels though, so make
waiting for idle a separate function and call it where necessary.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>