hmp: sched: Clean up hmp_up_threshold checks into a utility fn

hmp: sched: Clean up hmp_up_threshold checks into a utility fn

In anticipation of modifying the up_threshold handling, make all
instances use the same utility fn to check if a task is eligible
for up-migration. This also removes the previous difference in
threshold comparison where up-migration used '!<threshold' and
idle pull used '>threshold' to decide up-migration eligibility.
Make them both use '!<threshold' instead for consistency, although
this is unlikely to change any results.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Jon Medhurst <tixy@linaro.org>