x11-drivers/xf86-video-amdgpu: Add the amdgpu driver for Xorg. Currently and temporarily requires FreeBSDDesktop kernel fork of 12-CURRENT.

Reviewed by: rene (mentor), swills (mentor), kwm (x11@)
Approved by: rene (mentor), swills (mentor)
Differential Revision: https://reviews.freebsd.org/D9377