sched: use wakeup latency as c-state determinant

C-state aware scheduler at present, uses a raw c-state index number as
its determinant and avoids task placement on deeper c-state CPUs at
cost of latency.  However there are CPUs offering comparable wake-up
latency at different c-state levels and the wake-up latency at each
c-state levels are already have being fed to scheduler.

Hence use the wakeup_latency as c-state determinant instead of raw
c-state index to avoid unnecessary task packing where it's doable.

CRs-fixed: 1074879
Change-Id: If927f84f6c8ba719716d99669e5d1f1b19aaacbe
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>