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: I0ff90d1552daba7824a5d8a9e50ff8463de0d861
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>