modesetting: Validate the atom for enum properties

The client could have said anything here, and if what they said doesn't
actually name an atom NameForAtom() will return NULL, and strcmp() will
be unhappy about that.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit d4995a39)