media: vsp1: Use lockdep assertions to enforce documented conventions

A few functions have documented locking conventions. Documentation is
nice, but runtime checks are better. Enforce the conventions with
lockdep assertions.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>