sched: window-stats: Fix incorrect calculation of partial_demand

When using MAX_POLICY, partial_demand is calculated incorrectly as 0.
Fix this by picking maximum of previous 4 windows and most recent
sample.

Change-Id: I27850a510746a63b5382c84761920fc021b876c5
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>