Comment # 52 on bug 1004453 from Max Staudt

These errors look like what happens without "iomem=relaxed". No idea what KDM3
does differently, as starting the X server directly does work (as you
confirmed). Maybe you rebooted without "iomem=relaxed" by mistake?

As far as I can see, Xorg is now working for you, so let's close the bug.

This was quite an interesting problem, thanks for filing and assisting with the
bug!