media: ti-vpe: cal: Unregister video device before cleanup

In cal_remove(), unregister the video devices as the first operation,
before cleaning up the V4L2 objects, to avoid use-after-free. This isn't
a complete solution yet, as video nodes can be kept open across
unregistration.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>