opp: Reduce code duplication in _set_required_opps()

Move call to dev_pm_genpd_set_performance_state() to a separate
function so we can avoid duplicating the code for the single and
multiple genpd case.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
[ Viresh: Validate virtual device before use ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>