PM: sleep: Suspend async parents after suspending children

In analogy with the previous change affecting the resume path,
make device_suspend() start the async suspend of the device's parent
after the device itself has been processed and make dpm_suspend() start
processing "async" leaf devices (that is, devices without children)
upfront so they don't need to wait for the "sync" devices they don't
depend on.

On the Dell XPS13 9360 in my office, this change reduces the total
duration of device suspend by approximately 100 ms (over 20%).

Suggested-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/3541233.QJadu78ljV@rjwysocki.net