sched: update depth before enqueue entity

When task is switching from rt to normal, depth is not
updated before __enqueue_entity. So it got chance when
in the process of pick next task, checking wrong depth
and get a NULL parent for proceeding.

Change-Id: Ie1052cbb965cf0c899e623ab773f4344c68bbf40
Signed-off-by: Maria Yu <aiquny@codeaurora.org>