sched/walt: Fix stale window start marker passed to the schedutil

With commit d8c5bfcc ("sched: Make sure window start passed to
schedutil is consistent"), the rq->load_reported_window is presented
to the governor as the window_start marker. The rq->load_reported_window
is updated when load is reported to governor only during a window rollover.
So it should be consistent with the current window start mark. But for a
just hotplugged in CPU, the rq->load_reported_window is not updated
until the next window rollover.

If the load is reported for any other reason before the next window
rollover, the window start marker passed to the schedutil would be
stale and leads to a BUG_ON() in schedutil. The recent window start marker
is cached in WALT in walt_irq_work_lastq_ws. Use this instead of
load_reported_window to fix this problem.

The rq->window_start is cached in rq->load_reported_window to filter
the utilization updates in the same window. This is not needed since
utilization updates are not sent when SCHED_CPUFREQ_WALT flag is not
set. So kill the load_reported_window maintenance.

Change-Id: Idaefcb0b9cecb15ea436ac7a66cb6da81e3852a1
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>