After disabling and reenabling the CRTC the DCU sometimes got stuck
displaying the whole screen with a solid color. Disabling and
reenabling the CRTC did not recover from the situation. This was
often reproducable by just restarting the X-Server.

The disabling sequence is not explicitly documented. But it turns
out that disabling the planes before disabling the CRTC seems to
prevent the above situation from happening.

Use the callback ->atomic_disable instead of ->disable which allows
to use the drm_atomic_helper_disable_planes_on_crtc() helper to
disable planes before disabling the controller.

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