soc: amlogic: meson-gx-pwrc-vpu: fix error on shutdown when domain is powered off

When operating the system headless headless, the domain is never
powered on, leaving the clocks disabled. The shutdown function then
tries to disable the already disabled clocks, resulting in errors.
Therefore call meson_gx_pwrc_vpu_power_off() only if domain is
powered on.
This patch fixes the described issue on my system (Odorid-C2).

Fixes: 339cd0ea "soc: amlogic: meson-gx-pwrc-vpu: fix power-off when powered by bootloader"
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>