sched: ceil idle index to prevent from out of bound accessing

It's possible size of given idle cost index is smaller than CPU's
possible idle index size.  Ceil the CPU's idle index to prevent out
of bound accessing.

Change-Id: Idecb4f68758dd0183886ea74d0e9da3d236b0062
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>