sched: Do eas idle balance regardless of the rq avg idle value

EAS relies on idle balance to migrate a misfit task towards a cpu with
higher capacity.

When such a cpu becomes idle, idle balance should happen even if the rq
avg idle is smaller than the sched migration cost (default 500us).

The rq avg idle is updated during the wakeup of a task in case the rq has
a non-null idle_stamp. This value stays unchanged and valid until the next
task wakes up on this cpu after an idle period.

So rq avg idle could be smaller than sched migration cost preventing the
idle balance from happening. In this case we would be at the mercy of
wakeup, periodic or nohz-idle load balancing to put another task on this
cpu.

To break this dependency towards rq avg idle make EAS idle balance
independent from this rq avg idle has to be larger than sched migration
cost.

Change-Id: I880a25180062444d72947461d976dc44f9672f13
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>