sched: Don't active migrate tasks to CPUs in the same cluster

At the tick the scheduler checks whether a running task needs to
be migrated for some reason. If the reason is to up or down migrate
a task, we should only kick active balance when select_best_cpu()
returns a CPU in the appropriate cluster. Active migrating a task
to another CPU in the same cluster is wasteful.

Change-Id: I2c899a9c2dfb8ec4f983d7e355f0960f88bd3b45
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>