sched/core_ctl: disable prev assist by default

Prev assist is not needed for all clusters with in the system
and can be enabled from user space for specific cluster as needed.
So, disable prev assist by default and update checks as necessary.

Change-Id: I3e3770a0f2dc6f588693c98e3b8514b4fab29fd6
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>