https://bugs.freedesktop.org/show_bug.cgi?id=81382
--- Comment #25 from Alex Deucher agd5f@yahoo.com --- (In reply to comment #24)
Apply them both together.
Well, assuming your laptop has the same pci ids as Patrick. If not, just append radeon.backlight=0 to the kernel command line in grub and attach your lspci -vnn output.