cpufreq: schedutil: Ignore CPU load older than WALT window size

The old code used to ignore CPU load older than 1 tick. Since the WALT
window size can be more than 1 tick (and is configurable), this can cause
an incorrect frequency drop in the middle of a WALT window. Use WALT window
size to ignore CPU load instead.

Change-Id: If42420f595ba9722866174c2cda8b6c2fdf34f92
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>