I have several issues trying to connect a 4k display to the radeon card via
HDMI (@30Hz). If I boot with the screen connected, when trying to take over
modesetting from EFI, the internal notebook display goes black as well as the
external.

When I try to attach it during runtime, I can see the 3840 mode, but when I
tryt to select it I get 
[   97.667521] [drm:atom_op_jump [radeon]] *ERROR* atombios stuck in loop for
more than 5secs aborting
[   97.667549] [drm:atom_execute_table_locked [radeon]] *ERROR* atombios stuck
executing BF64 (len 962, WS 0, PS 0) @ 0xC1A1

Switching back to 1920 works fine, trying again simply causes the same error
again. Tested Linux kernels 4.1.0 and 3.19. Ubuntu 15.04.

I might not be able to test for long, will probably send the screen back before
long if I can't get it working in full resolution at all.