OPP: Switch to use dev_pm_domain_set_performance_state()

To support performance scaling for any kinds of PM domains, let's move away
from using the genpd specific API, dev_pm_genpd_set_performance_state(), to
the common dev_pm_domain_set_performance_state().

No intended functional impact at this point.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>