PM / Domains: Factorize dev_pm_genpd_set_performance_state()

Separate out _genpd_set_performance_state() and
_genpd_reeval_performance_state() from
dev_pm_genpd_set_performance_state() to handle performance state update
related stuff. This will be used by a later commit.

Tested-by: Rajendra Nayak <rnayak@codeaurora.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>