On a PowerPC machine, when Open Firmware Frame Buffer is used (OFfb), I
cannot `modprobe radeonfb` (but I can load `radeon`). It currently fails with:

[   96.551486] radeonfb 0000:00:10.0: enabling device (0006 -> 0007)
[   96.551526] radeonfb 0000:00:10.0: BAR 0: can't reserve [mem
0x98000000-0x9fffffff pref]
[   96.551531] radeonfb (0000:00:10.0): cannot request region 0.
[   96.551545] radeonfb: probe of 0000:00:10.0 failed with error -16

It seems (to me) that it should be possible to add something like this
to `radeonfb`:

https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/radeon/radeon_drv.c#L353

[...]
        /* Get rid of things like offb */
        ret = radeon_kick_out_firmware_fb(pdev);
        if (ret)
[...]