drm/imx: move arming of the vblank event to atomic_flush

Right now the vblank event completion is racing with the atomic update,
which is especially bad when the PRE is in use, as one of the hardware
issue workaround might extend the atomic commit for quite some time.

If the vblank IRQ happens to trigger during that time, we will prematurely
signal the atomic commit completion to userspace, which causes tearing
when userspace re-uses a framebuffer we haven't managed to flip away from
yet.

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