https://bugs.freedesktop.org/show_bug.cgi?id=82588
--- Comment #21 from Mike Lothian mike@fireburn.co.uk --- Yes 3.17-rc5
I downloaded the firmware files when they were first made available back when drm-next first added the support for the new firmware at the end of the 3.16 cycle
Those files must have been damaged - installing the latest linux-firmware (or radeon-ucode) fixed this for me
You are probably suffering from a different issue and should raise a separate bug