PM / OPP don't match for existing OPPs when list is empty

OPP list is guaranteed to be empty when 'dev_opp' is created. And so we don't
need to run the comparison loop with existing OPPs.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>