psi: track changed states

Introduce changed_states parameter into collect_percpu_times to track
the states changed since the last update.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Change-Id: If123f39853b674c67421689f36e5a50bf73c9096
Patch-mainline: linux-mm @ 03/19/2019, 23:56
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>