PM: domains: Measure suspend/resume latencies in genpd based on governor

The QoS latency measurements for devices in genpd_runtime_suspend|resume()
are superfluous, unless the corresponding genpd has a governor assigned to
it, which would make use of the data.

Therefore, let's improve the behaviour in genpd by making the measurements
conditional, based upon if there's a governor assigned.

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