gpu: ipu-v3: add unsynchronised DP channel disabling

When disabling the foreground DP channel during a modeset, the DC is
already disabled without waiting for end of frame. There is no reason
to wait for a frame boundary before updating the DP registers in that
case.
Add support to apply updates immediately. No functional changes, yet.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>