Can you try booting with radeon.new_pll=0 added to your kernel command line (not at the same time as the radeon.modeset=0 request above) to see if it fixes the issue also?
Can you try booting with radeon.new_pll=0 added to your kernel command line (not at the same time as the radeon.modeset=0 request above) to see if it fixes the issue also?