drm/fsl: Drop drm_vblank_cleanup

Again cleanup before irq disabling doesn't really stop the races,
so just drop it. Proper fix would be to put drm_atomic_helper_shutdown
before everything gets cleaned up.

Cc: Stefan Agner <stefan@agner.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>