media: rkisp1: isp: Fix and simplify (un)registration

The rkisp1_isp_register() and rkisp1_isp_unregister() functions don't
destroy the mutex (in the error path for the former). Fix this, simplify
error handling at registration time as media_entity_cleanup() can be
called on an uninitialized entity, and make rkisp1_isp_unregister() and
safe to be called on an unregistered isp subdev to prepare for
simplification of error handling at probe time.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>