Just pulled from airlied's drm-next branch about 30 minutes ago. And I'm still getting a blank screen when X starts with KMS. I get the high res framebuffer so I know KMS does work. If I disable KMS, X starts just fine.
I finally decided to ssh into my machine and check out the xorg log.
Here is a pastebin of Xorg.0.log:
http://pastebin.com/m6a37e57b
Edit:
Here are the errors:
(EE) RADEON(0): [dri] RADEONDRIGetVersion failed because of a version mismatch.
(EE) RADEON(0): Acceleration initialization failed
And the warnings:
(WW) Open ACPI failed (/var/run/acpid.socket) (No such file or directory)
(WW) RADEON(0): Direct rendering disabled
I only get those errors on KMS, 3D and everything works fine without it.