Triggers+autonomous transactions = recipe for disaster and the wrong way to go.
You should rethink what it is you're trying to accomplish.
Probably a COMMIT would get you past the deadlock error (if I recall correctly), but then your data can get corrupted, as you are messing with transactions.
Probably a procedural approach would be best.
Triggers have several disadvantages (slow-by-slow processing, performance, 'automagical' stuff happening), as you now are discovering.
Consult the Online Oracle Documentation regarding autonomous transactions and the deadlock error @
And for some more background: