opp: Make dev_pm_opp_set_rate() handle freq = 0 to drop performance votes

For devices with performance state, we use dev_pm_opp_set_rate() to set
the appropriate clk rate and the performance state.

We do need a way to remove the performance state vote when we idle the
device and turn the clocks off. Use dev_pm_opp_set_rate() with freq = 0
to achieve this.

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
[ Viresh: Updated _set_required_opps() to handle the !opp case ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>