sched: walt: move the cumulative window demand updates to enqueue

Move the cumulative window demand updates/fixups from the task
wakeup to enqueue path. This refactoring is needed for the
subsequent patch that adds CFS_BANDWIDTH support to walt.
For example, task wakeup path does not know about a throttled
task moving to an unthrottled cgroup.

Change-Id: I63f7ccacfe98864ee5a13e747190a7177b5dc128
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>