sched/fair: ignore backup CPU when not valid

The find_best_target can sometimes not return a valid backup CPU, either
because it cannot find one or just becasue it returns prev_cpu as a backup.
In these cases we should skip the energy_diff evaluation for the backup CPU.

Change-Id: I3787dbdfe74122348dd7a7485b88c4679051bd32
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
(cherry picked from commit d9bcf5b88594a0225b51878236e49305f272eadc)
[trivial cherry-pick issue]
Signed-off-by: Quentin Perret <quentin.perret@arm.com>