sched/fair: Use EAS also when overutilized

Keep looking for an energy efficient CPU even when the system is
overutilized and use the CPU returned by feec() if it has been able to find
one. Otherwise fallback to the default performance and spread mode of the
scheduler.
A system can become overutilized for a short time when workers of a
workqueue wake up for a short background work like vmstat update.
Continuing to look for a energy efficient CPU will prevent to break the
power packing of tasks.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>