PM: domains: Split code in dev_pm_genpd_set_performance_state()

To prepare some of the code in dev_pm_genpd_set_performance_state() to be
re-used from subsequent changes, let's split it up into two functions.

Reviewed-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>