PM: domains: Trust domain-idle-states from DT to be correct by genpd

If genpd has parsed the domain-idle-states from DT, it's reasonable to
believe that the parsed data should be correct for the HW in question.
Based upon this, it seem superfluous to let genpd measure the corresponding
power-on/off latencies for these states.

Therefore, let's improve the behaviour in genpd by avoiding the
measurements for the domain-idle-states that have been parsed from DT.

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