cpufreq: sched: WALT: don't apply capacity margin twice

With WALT all the scheduler classes' load are accounted in scr->cfs and
update_cpu_capacity_request() adds capacity margin.  At present, at tick
path, scheduler also adds capacity margin.  Therefore the margin applied
twice.

Fix such error by using margin applied cpu utilization only for checking
whether frequency increase is needed.

Change-Id: Id7d8cc73b2e4eec70b274ca66e09bb0b16bf6f09
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
(trivial rebase conflict)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>