psi: Use deferrable psi_avgs_work

The logic for psi_avgs_work to not reschedule itself does not appear
to work properly.

1) psi_avgs_work detects itself as a running task due to accounting for
current states in get_recent_times().

2) Drivers with self-rescheduling deferrable work with a period
less than that of psi_avgs_work will always run when psi_avgs_work runs.
This causes psi_avgs_work to retrigger.

Make psi_avgs_work deferrable to avoid this loop.

Change-Id: I90edc5d3cce87ada803f549f417cd0108ef2017f
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>