sched/fair: Avoid force newly idle load balance if have iowait task

If this runqueue have iowait task and still force newly
idle load balance, will make iowait task performance
worse. Need to do the average idle time check if
there is iowait task on this runqueue to have upcoming
iowait task have enough cpu resource.

Change-Id: I8e4dd440edbbd14d5170fbcf358bdaf5cc5b26e8
Signed-off-by: Maria Yu <aiquny@codeaurora.org>