sched: clean up idle task's mark_start restoring in init_idle()

The idle task's mark_start can get updated even without the CPU being
online. Hence the mark_start is restored when the CPU is coming online.

The idle task's mark_start is reset in init_idle()->__sched_fork()->
init_new_task_load(). The original mark_start is saved and restored
later. This can be avoided by moving init_new_task_load() to
wake_up_new_task(), which never gets called for an idle task.

We only care about idle task's ravg.mark_start and not initializing
the other fields of ravg struct will not have any side effects.

This clean up allows the subsequent patches to drop the rq->lock
while calling __sched_fork() in init_idle().

CRs-Fixed: 965873
Change-Id: I41de6d69944d7d44b9c4d11b2d97ad01bd8fe96d
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>