sched: Take downmigrate threshold into consideration

If the tasks are run on the higher capacity cluster solely due to the
reason that they can not be be fit in the lower capacity cluster, the
downmigrate threshold prevents the frequent tasks migrations between
the clusters.

Change-Id: I234a23ffd907c2476c94d5f6227dab1bb6c9bebb
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>