https://bugs.freedesktop.org/show_bug.cgi?id=78297
--- Comment #9 from Christian König deathsimple@vodafone.de --- That's the problem:
[ 1.714537] [drm] Detected VRAM RAM=1024M, BAR=128M
Your PCI BAR is smaller than usually, for a REDWOOD 256M are normal. Going to hack up a patch for this.