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

The rkisp1_params_register() and rkisp1_params_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_params_unregister() safe to be called on an unregistered params
node 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>