Created attachment 131532 [details]
"dmesg | grep drm" output (4.12.rc2, libdrm 2.4.8)

I updated libdrm and xorg from edgers ppa, and drm-tip kernel is now able to
boot.
I attached latest "dmesg | grep drm" output. I went trough logs while changing
drm.debug mask, but have failed to see any errors.  

Package versions are:
libdrm-radeon1/artful,now 2.4.80+git20170524.eeb51df3-0ubuntu0ricotz,
4.12.0-994-lowlatency kernel,
xorg 1:7.7+19 with xserver-xorg-video-radeon 1:7.9.0-0ubuntu1