sched/fair: do not pull single task if a dst_cpu is not idle

Misfit task will get migrated in the tick path so there is no need
to do active pulling of the single task.

Change-Id: I372b23c038f4e34af4608859ba7a7d9ec9ca4283
Signed-off-by: Runmin Wang <runminw@codeaurora.org>