sched: optimize select_best_cpu() where !sched_enable_power_aware

All CPUs power costs are fixed as its capacity when EA (energy aware
scheduler) isn't enabled.  Thus select_best_cpu() doesn't need to
calculate power cost delta to check if power delta exceeds when EA is off.
Optimize select_best_cpu() with precalculated min/max capacity delta
percentage value.

CRs-fixed: 849655
Change-Id: Ib6a94ac2d9a62c9a7be4bb682092a16aea438459
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>