drm: Move vblank cleanup from unregister to release

Calling drm_vblank_cleanup() in drm_dev_unregister() causes issues with
drivers that have moved away from the .load() and .unload() midlayer.
Those drivers call drm_dev_unregister() as the first operation at unbind
time, before shutting down the device. This results in warnings due to
drm_vblank_cleanup() being called with vblank interrupts still active,
and then to vblank events being sent after cleanup.

Fix the problem by moving vblank cleanup from drm_dev_unregister() to
drm_dev_release() that is guaranteed to be called after drivers shut
down the device.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>