PM: sleep: Move some assignments from under a lock

The async_error and pm_transition variables are set under dpm_list_mtx
in multiple places in the system-wide device PM core code, which is
unnecessary and confusing, so rearrange the code so that the variables
in question are set before acquiring the lock.

While at it, add some empty code lines around locking to improve the
consistency of the code.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>