drm/imx: track flip state explicitly

Start tracking the flip state explicitly, as opposed to inferring
it from the presence if a new FB. This is a preparatory step to
introduce an new immediate state, where we can wait for a fence to
signal.

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