imx-drm: ipu-dp: split disabling the DP foreground channel from disabling the DP module

The former has to be done before disabling the DMFC, the latter has to be
done afterwards. Otherwise the DMFC FIFOs never get cleared properly.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>