gpu: ipu-v3: prg: wait for double buffers to be filled on channel startup

Wait for both double buffer to be filled when first starting a channel.
This makes channel startup a lot more reliable, probably because it allows
the internal state machine to settle before the requests from the IPU are
coming in.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
[p.zabel@pengutronix.de: rebased before switch to runtime PM]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>