UPSTREAM: PM / sleep: make PM notifiers called symmetrically

(cherry picked from commit ea00f4f4)

This makes pm notifier PREPARE/POST symmetrical: if PREPARE
fails, we will only undo what ever happened on PREPARE.

It fixes the unbalanced CPU hotplug enable in CPU PM notifier.

Change-Id: I01dce3cc95c5d6b8913b7b6be301f2909258c745
Signed-off-by: Lianwei Wang <lianwei.wang@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>