drm/imx: notify DRM core about CRTC vblank state

Make sure the DRM core is aware that there will be no vblank interrupts
incoming if the CRTC is disabled. That way the core will reject any
attempts from userspace to wait on a vblank event on a disabled CRTC.

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