sched: Per-cpu prefer_idle flag

Remove the global sysctl_sched_prefer_idle flag and replace it with a
per-cpu prefer_idle flag. The per-cpu flag is expected to same for all
cpus in a cluster. It thus provides convenient means to disable
packing in one cluster while allowing packing in another cluster.

Change-Id: Ie4cc73bb1a55b4eac5697be38e558546161faca1
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>