Note: when re-enabling the radeon card using vgaswitcheroo (echo ON > /sys/kernel/debug/vgaswitcheroo/switch) the bug doesn't appear, so this is probably related to vgaswitcheroo disabling the card and the radeon driver not being aware of that fact (this worked in Ubuntu 12.04 though).
Note: when re-enabling the radeon card using vgaswitcheroo (echo ON > /sys/kernel/ debug/vgaswitch eroo/switch) the bug doesn't appear, so this is probably related to vgaswitcheroo disabling the card and the radeon driver not being aware of that fact (this worked in Ubuntu 12.04 though).