PM: sleep: Rearrange dpm_async_fn() and async state clearing

In preparation for subsequent changes, move the power.completion
reinitialization along with clearing power.work_in_progress into a
separate function called dpm_clear_async_state() and rearrange
dpm_async_fn() to get rid of unnecessary indentation.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/8494650.T7Z3S40VBb@rjwysocki.net