media: rkisp1: Simplify rkisp1_entities_register() error path

Now that all the unregistration functions are safe to call on
non-registered entities, the error path in rkisp1_entities_register()
can be simplified. Factor out the unregistration to a separate function
to share code with rkisp1_remove().

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>