Iq drop from martial arts/fighting, possible?

Yes: Yes, traumatic brain injury can result in decrease in iq. It results from blows to the head. Repeated head injury worsens this.

Yes. Which Style?: Yes. Obviously, any martial art or fighting that involves contact with the head can lead to concussion, traumatic brain injury, etc. There may be some styles which may be safer, such as Tai Chi & Aikido. These arts rarely have percussive impact. It all depends upon how you train.