sched/walt: don't account CPU idle exit time to task demand

Discount the CPU idle exit time from the first task picked up to
run on an idle CPU.

Change-Id: Iaca5f7c022fbffeaa288ed09928fe34ac8bf5b9f
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
[satyap@codeaurora.org: trivial merge conflict resolution]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>