sched/fair: enforce EAS mode

For non latency sensitive tasks the goal is to optimize for energy efficiency.
Thus, we should try our best to avoid moving a task on a CPU which is then
going to be marked as overutilized.

Let's use the capacity_margin metric to verify if a candidate target CPU
should be considered without risking to bail out of EAS mode.

Change-Id: Ib3697106f4073aedf4a6c6ce42bd5d000fa8c007
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
(cherry picked from commit f95753da4ba7e1c1eee0500ffe41b3e5fa68b347)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>