drm/vc4: vec: Switch to devm_pm_runtime_enable

devm_pm_runtime_enable() simplifies the driver a bit since it will call
pm_runtime_disable() automatically through a device-managed action.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220711173939.1132294-62-maxime@cerno.tech