sched/walt: Check if necessary for sched_boost modify

Check if necessary to do exit and next_boost enter
oprations instead of doing it regardlessly.
Core control boost refcounts has been wrong if doing
current sched_boost exit and next_boost enter by mistake.

Change-Id: Ie40ebf0be0e1b99291d07d160ebef5964d44ba16
Signed-off-by: Maria Yu <aiquny@codeaurora.org>
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>