imx-drm: ipu-dmfc: wait for FIFOs to run empty before disabling

Disabling the DMFC module while there is still data in the FIFOs could
cause the "new frame before end of frame" error state when the DMFC is
enabled again.

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