PM / OPP: set new_opp->dev_opp to a valid dev_opp

We find/allocate dev_opp after using its value to fill new_opp->dev_opp right
now. Move this to a later point where dev_opp is valid.

Fixes: a7470db6 (PM / OPP don't match for existing OPPs when list is empty)
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>