sched: Make sure window start passed to schedutil is consistent

The scheduler sends some notifications for individual CPUs. If these
events happen close to the window boundary, the window_start passed
to schedutil may go out of order.

Change-Id: Ib6338bd4bc3b1c41c424ec959056c57c0ed7507e
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>