https://bugs.freedesktop.org/show_bug.cgi?id=107072
--- Comment #10 from Alex Deucher alexdeucher@gmail.com --- (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).