imx-drm: imx-dp: when disabling the DP foreground channel, wait until the DP...

imx-drm: imx-dp: when disabling the DP foreground channel, wait until the DP background channel is finished before disabling the IDMAC channel

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