core_ctl: Update cluster->active_cpus in eval_need()

The cluster->active_cpus is not updated in eval_need(). The new need
for CPUs is compared against the previous cluster->active_cpus. If
another client isolates a CPU, cluster->active_cpus becomes stale and
we fail to detect the change in need for CPUs.

Change-Id: Ib58b8f0bd03dd2b4a174de2ac54eb0c60c59f9f7
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>