sched/fair: make upmigration of big tasks possible under boost scenarios

upmigration of big tasks is broken right now when boost is set as
we skip checking for migration thresholds when task is running on
an intermediate CPU. Fix it by updating the check.

Change-Id: I84de520ef7998e5fab26734cc3d20fc314fdca8b
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>