I realise 'who saw you' is probably more common, but is 'who did see you' wrong? This is with respect to why who-questions don't need 'do'-support as an aux verb.
Word order with question word as subject (from PEU by Michael Swan):
When who, which, what or whose is the subject (or part of the subject), the question word comes before the verb, and do cannot normally be used.
Who left the door open? ( NOT Who did leave...?)
Which type of battery lasts longest? (NOT Which type of battery does last longest?)
What happened? (NOT What did happen?)
But do can be used after a subject question word for emphasis, to insist on an answer.
Well, tell us - what did happen when your father found you?
Who saw you?
Who did you see?
mean completely different things.
In Who saw you?, who is the subject of the sentence, so the word who is in the nominative case.
In Who did you see?, who is actually the direct object of the sentence, so who is in the accusative case. You can understand this better if you convert the sentence like this:
You saw who?
One more thing worth telling: in Who did you see?, who is used in an informal manner, because the right form of the word who in the accusative case in formal English is whom.
So you sentence must actually be like this:
Whom did you see?