PM / Domains: Improve warn for multiple states but no governor

It's possible a PM domain defines only one state and it does not need
a governor to work. For such case, a warning actually is not necessary.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>