It had a copy and paste mistake that I didn't notice. :/
It was using the CreatePixmapReq.
Also add a missing B16 to the end of the length for the DestroyPixmapReq struct.
Now the AppleDRIDestroyPixmap request seem to work.
(cherry picked from commit 295fe25bd8fa2d141291a9d9b6ef7b75fcccb4dd)