PM: domains: Measure power-on/off latencies in genpd based on a governor

The measurements of the power-on|off latencies in genpd for a PM domain 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>