intel_pstate: Make use of `balance_power` preference

Currently when using intel_pstate, p-p-d is only able to set performance, balance_performance and power EPPs. This is very limiting, since on some devices there's a huge difference in performance and responsiveness between balance_power and power, while there's a noticeable difference in battery life with smaller performance impact between balance_performance and balance_power.

Ideally, I'd like my system to switch automatically between balance_performance and balance_power when (dis)connecting AC power (which could be done either by p-p-d or by external daemon, like G-S-D or PowerDevil), while switching to performance or power would be done manually by me by selecting appropriate option from the menu based on intended workload (power is useful when reading or watching something in train, for instance).

In the past balance_power was actually used when powered from a battery, but it got removed in !63 (merged). Since getting it back is what would make p-p-d actually useful for use cases like mine (currently I have to bypass it completely), it may be a good idea to think about how could it fit into p-p-d again.