Yes the info is inadequate. the op notes is very important.
the supra tentorial dermoid cyst / or any dermoid cyst does not have an accurate code.
Dermoid can occcur in ovary, skin , spinal cord and the brain, the brain Dermoid are the rarest amongst them all.
Though the approach was posterior auricular, from the notes you furnished it seems to be a Supratentorial Dermoid cyst of the brain usually occuring at the frontal base or temporal base/posterior cranial fossa
so I would use the "Benign Neoplasm of the Brain and the other parts of the nervous system - 225.0 Brain , benign neoplasm.
you can still give the symptoms like varient headache syndrome: 346.2. Headache: 784.0 or siezure if it was there, as sec diagnosis, which were the reason for the encounter.
I doubt your diagnosis Code 216.x seems to be off the track, because the basic structural pathology of Dermoid , is not found in any one of those listed in 216.2.