PM / OPP: Create _remove_device_opp() for freeing dev_opp

This will be used from multiple places later. Lets create a separate
routine for that.

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 3bac42caec612a1b82db7944570353cddca6a013)
Signed-off-by: Alex Shi <alex.shi@linaro.org>