sched/fair: use tunable value for capacity margin checks

As user can change the migration thresholds, need to use the
upmigration threshold for capcity margin checks instead of
hard coded one to avoid undesired behavior.

Change-Id: I83e68bbaeba814c720af5e6ae4c37b4162141c51
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>