sched/fair: Add EAS_USE_NEED_IDLE feature

The schedtune.prefer_idle flag makes scheduler to look for an idle
CPU across all clusters. Where as the need_idle flag limits the
search to the lower capacity cluster which strikes a better
balance between power and performance compared to prefer_idle.

Add EAS_USE_NEED_IDLE feature, when enabled need_idle path is
enforced for tasks that has schedtune.prefer_idle set.

Change-Id: Ic83cd9f8bae1112efa4e62de3c60a86ba9c777d7
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>