gpu: ipu-v3: prg: add function to get channel configure status

This allows channels using the PRG to check if a requested configuration
update has been applied or is still pending.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
[p.zabel@pengutronix.de: inverted logic: done -> pending]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>