sched: Avoid active migration of small tasks

We currently check the need to migrate the currently running task in
scheduler_tick(). Skip that check for small tasks, as its not worth
the effort!

Change-Id: Ic205cc6452f42fde6be6b85c3bf06a8542a73eba
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>