Actually, I was just having a look at Warlocks again. In the PHB, in the description of the Great Old One patron/subclass, they give examples of patrons : Ghaunadaur, Tharizdun, Dendar, Zargon and ... Great Cthulhu. So whatever happened between TSR and Chaosium back in the days, WotC does seem to have the right to explicitly use Cthulhu.
