opp: Make _set_opp_custom() work without regulators

Check whether OPP table has regulators in _set_opp_custom() and set up
dev_pm_set_opp_data accordingly. Now _set_opp_custom() works properly,
i.e. it doesn't crash if OPP table doesn't have assigned regulators.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
[ Viresh: Rearrange the routine a bit ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>