sched: WALT: fix broken cumulative runnable average accounting

When running tasks's ravg.demand is changed update_history() adjusts
rq->cumulative_runnable_avg to reflect change of CPU load.  Currently
this fixup is broken by accumulating task's new demand without
subtracting the task's old demand.

Fix the fixup logic to subtract the task's old demand.

Change-Id: I61beb32a4850879ccb39b733f5564251e465bfeb
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
(cherry picked from commit 48f67ea85de468a9b3e47e723e7681cf7771dea6)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>