I suspect it could be the same problem as in bug #108514.

Best workaround known would be to boot with nomodeset kernel parameter.

Adding drm.debug=4 (kernel mode setting messages) to kernel parameters
would give more usefull dmesg output.

Suspected problem for #108514 would be related to compute_avivo_pll_[something]
in gpu/drm/radeon/radeon_display.c.
Possibly the max pll clock frequency.

I hope to be able to investigate beginning of next week.