drm/imx: ipuv3-plane: add function to query atomic update status

This function allows upper layer to check if a requested atomic update
to the plane 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>