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

The rkisp1_rsz_register() and rkisp1_rsz_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_rsz_unregister() and
rkisp1_resizer_devs_unregister() safe to be called on an unregistered
resizer subdev to prepare for simplification of error handling at probe
time.

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