sched/fair: use utilization to select misfit task

utilization is used to detect a misfit task but the load is then used to
select the task on the CPU which can lead to select a small task with
high weight instead of the task that triggered the misfit migration.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Valentin Schneider <valentin.schneider@arm.com>