(In reply to Pontus Gråskæg from comment #9)
> Looking at your dmesg output, it looks like you might have hit this
> bug/issue: bug 105880
> 
> "[dc] No support for LVDS or VGA connectors (Cannot find any crtc or sizes)"
> 
> AMD/Harry Wentland/Alex Deucher have commits in the mainline kernel to
> disable the DC code by default for Bonaire, Kabini, Kaveri, and Mullins as
> certain hardware interfaces are not supported in the DC code. Your cpu (from
> dmesg) is an AMD A8-7650K Radeon R7, codename Kaveri.

Unless you are using VGA or LVDS connectors this is a different issues.  That
does not seem to be the case here (HDMI and DVI are supported in the DC code).