drm/imx: implement fence sync

If the FB is backed by a GEM object with an dma-buf attached
we need to wait for any pending fences to signal before executing
the page flip.

The implementation is straight forward by deferring the flip to
a workqueue in that case.

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