[MERGED] pmdomain: rockchip: cleanup mutex handling in rockchip_pd_power

Use the cleanup infrastructure to handle the mutex, which
slightly improve code readability for this function.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Adrian Larumbe <adrian.larumbe@collabora.com> # On Rock 5B
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>