thermal: core: Move power_actor_set_power into IPA

Since the power actor section has one function power_actor_set_power()
move it into Intelligent Power Allocation (IPA). There is no other user
of that helper function. It would also allow to remove the check of
cdev_is_power_actor() because the code which calls it in IPA already does
the needed check. Make the function static since only IPA use it.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201015112441.4056-5-lukasz.luba@arm.com