Comment # 2 on bug 964837 from Stefan Dirsch

8.865756] [drm] VGACON disable radeon kernel modesetting.
8.865782] [drm:radeon_init [radeon]] *ERROR* No UMS support in radeon module!

1. radeon X driver no longer supports non-KMS (=UMS), you need to enable KMS.
Please proovide dmesg output when booting without 'nomodeset' boot option and
adding debug option 'drm.debug=0xe' instead. 

2. It's weird to see that no vga option is given here. I guess a suitable mode
even with using fbdev X driver may be possible. Check with 

  hwinfo --framebuffer 

which modes are availble in your VBIOS. Use one that suits your needs by
specifying boot option

  vga=0xXXXX

2) is independant from 1) and should be tested separately.