sched/fair: Add energy_diff dead-zone margin

sched/fair: Add energy_diff dead-zone margin

It is not worth the overhead to migrate tasks for tiny insignificant
energy savings. To prevent this, an energy margin is introduced in
energy_diff() which effectively adds a dead-zone that rounds tiny energy
differences to zero. Since no scale is enforced for energy model data
the margin can't be absolute. Instead it is defined as +/-1.56% energy
saving compared to the current total estimated energy consumption.

Change-Id: I6be069c752c701fb825430896b3b768a7ab2fee4
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
[rebase: on top of msm-google/android-msm-marlin-3.18,
         massage original patch which changes code in energy_diff()
	 into __energy_diff() introduced by SchedTune]
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
(cherry picked from commit 780cb5a5fa47adf13d4fc2b77e8e94448cd56098)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>