sched: don't select an inactive/isolated CPU in sbc()

In select_best_cpu(), if no CPU is selected from the candidate
cluster, the search is expanded to the backup cluster. The
current code may select an inactive/isolated CPU in the
backup cluster. Fix this.

Change-Id: Id1e8a2b2f84ea274cdeda408957490ca05ef5fdb
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>