PM: Do not use dynamically allocated objects in pm_wakeup_event()

Originally, pm_wakeup_event() uses struct delayed_work objects,
allocated with GFP_ATOMIC, to schedule the execution of pm_relax()
in future.  However, as noted by Alan Stern, it is not necessary to
do that, because all pm_wakeup_event() calls can use one static timer
that will always be set to expire at the latest time passed to
pm_wakeup_event().

The modifications are based on the example code posted by Alan.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>