PM: EM: Refactor em_pd_get_efficient_state() to be more flexible

Prepare the em_pd_get_efficient_state() for the upcoming changes and
make it possible to re-use. Return an index for the best performance
state. The funtion arguments that are introduced should allow to
work on different performance state arrays.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>