PM: domains: Return early if perf state is already set for the device

When dev_pm_genpd_set_performance_state() gets called to set a new
performance state for the device, let's take a quicker path by doing an
early return, if it turns out that the new state is already set for the
device.

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