sched/fair: Respect wake to idle over sync wakeup

Sync wakeup currently takes precedence over wake to idle flag. A sync
wakeup causes a task to be placed on a non-idle CPU because we expect
this CPU to become idle very shortly. However, even though the sync flag
is set there is no guarantee that the task will go to sleep right away
As a consequence performance suffers.

Fix this by preferring an idle CPU over a potential busy cpu when both
wake to idle and sync wakeup are set.

Change-Id: I6b40a44e2b4d5b5fa6088e4f16428f9867bd928d
CRs-fixed: 794424
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>