gpu: ipu-v3: remove IRQ dance on DC channel disable

This has never worked properly, as the IRQ got retriggered immediately
on unmask. Remove the IRQ wait dance, as it is apparently safe to disable
the DC channel at any point in time.

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