media: imx: Clean capture unregister

No locking is needed to call video_unregister_device(). Drop it.

Also, drop the superfluous video_is_registered() call, which is
done by video_unregister_device(), and re-order media_entity_cleanup()
and video_unregister_device() calls.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>