drm/fsl-dcu: disable outputs before unloading driver

Make sure that all outputs are disabled before unloading the DRM
driver. Otherwise vblank handling is not shut down properly and
warnings such as this appear:
WARNING: CPU: 0 PID: 540 at drivers/gpu/drm/drm_irq.c:339 drm_vblank_cleanup+0x5c/0x94

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