OPP: Use opp_table->regulators to verify no regulator case

The value of opp_table->regulator_count is not very consistent right now
and it may end up being 0 while we do have a "opp-microvolt" property in
the OPP table. It was kept that way as we used to check if any
regulators are set with the OPP core for a device or not using value of
regulator_count.

Lets use opp_table->regulators for that purpose as the meaning of
regulator_count is going to change in the later patches.

Reported-by: Quentin Perret <quentin.perret@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>