PM / domains: Remove default_stop_ok() API

There are currently no need to export default_stop_ok() as an API,
instead let's keep it local to the PM domain governor.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>