drm/imx: only enable vblank IRQs when needed

The vblank IRQ is only needed to trigger page flip work, so we
might as well disable it when there is no work to do.

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