Comment # 45 on bug 109345 from Michel Dänzer

Reviewing the information, I'm afraid it's not clear that there's a bug here.

The description says the FirePro is the secondary card in a PCIe 1x slot,
whereas the R7 is in the PCIe 16x slot. Thus it seems pretty clear that without
explicit configuration, Xorg should be expected to come up on the R7. Arguably
it was actually the previous behaviour that was buggy.

Something like this in /etc/X11/xorg.conf should be enough to make Xorg use the
FirePro card:

Section "Device"
        Identifier "whatever"
        BusID   "PCI:5@4096:0:0"
EndSection