Comment # 89 on bug 76490 from Maxim Sheviakov

(In reply to Tobias Droste from comment #88)
> Doesn't fix it for me, it still locks up at boot with dpm enabled and the
> quirk removed.
> 
> [drm] initializing kernel modesetting (PITCAIRN 0x1002:0x6810 0x174B:0xE271
> 0x00)

Have you put the new firmware files to your initramfs/initrd? Check replies
above.