PM / core: Introduce dpm_async_fn() helper

When we want to execute device pm functions asynchronously, we'll
do the following for the device:

  1) reinit_completion(&dev->power.completion);
  2) Check if the device enables asynchronous suspend.
  3) If necessary, execute the corresponding function asynchronously.

There are a lot of such repeated operations here, in fact we can avoid
this. So introduce dpm_async_fn() to have better code readability and
reuse.

And use this function to do some cleanup.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>