gpu: ipu-v3: pre: add double buffer status readback

This allows the upper layers to check if a double buffer update has
been applied by the PRE 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>