imx-drm: ipu-dc: wait for DC_FC_1 / DP_SF_END interrupt

Wait for the DC Frame Complete or DP Sync Flow End interrupts
before disabling DC channels.

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