drm/imx: fix crtc vblank state regression

The atomic conversion lost the notification to let the DRM core
know about the current state of the CRTC vblank interrupts. This
regressed the ability of the core to reject page flip attempts
on currently disabled CRTCs. Add back the notifications.

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