Comment # 6 on bug 972126 from Stefan Dirsch

With that in (xorg-x11-server RPM changelog)

- Add support for a driver specific PCI IDs files supplementing
  what's in xf86VideoPtrToDriverList(). PCI ID lists may be held
  in /etc/X11/xorg_pci_ids (boo#972126).

you can add autoselection for your gfx card via

  echo "10026939" > /etc/X11/xorg_pci_ids/amdgpu.ids

which maps to format:

 <vendorid>>deviceid>
 <vendorid>>deviceid>

in /etc/X11/xorg_pci_ids/<driver>.ids. Hope this helps.