pmdomain: rockchip: cleanup mutex handling in rockchip_pd_power

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: 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>
Link: https://lore.kernel.org/r/20250220-rk3588-gpu-pwr-domain-regulator-v6-2-a4f9c24e5b81@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>