pm: device_runtime: Simplify mutex usage

Assuming that pm_device_state_set is synchronous it is possible to
simplify the mutex usage. Now there are two places where the lock is
held, one in the worqueue handler and other in pm_device_request to
cover the synchronous path. It is no longer needed held the lock in the
pm_device_state_set callback and not needed to wait on the conditional
variable after set the state in the synchronous path.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>