drm/fsl-dcu: disable vblank events on CRTC disable

Disable vblank events when CRTC gets disabled. This avoids an
external abort when entering suspend while disable_timer is still
active: On resume the timer might fire immediately and cause a
register access in fsl_dcu_drm_disable_vblank before clocks get
enabled by the resume function.

Signed-off-by: Stefan Agner <stefan@agner.ch>