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>

drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c

@@ -108,6 +108,7 @@ static int fsl_dcu_unload(struct drm_device *dev)
 {
 	struct fsl_dcu_drm_device *fsl_dev = dev->dev_private;
 
+	drm_crtc_force_disable_all(dev);
 	drm_kms_helper_poll_fini(dev);
 
 	if (fsl_dev->fbdev)