https://bugs.freedesktop.org/show_bug.cgi?id=106199
--- Comment #4 from Öyvind Saether oyvinds@everdot.org --- I have the exact same problem on Fedora 28 with kernels 4.16.4 and 4.17rc2 and amdgpu.dc=1 on who different systems, one with a RX 560 and one with a RX 580. Both use DP.
start computer, get lightdm, login to XFCE logout computer frozen
My solution as of now is to use amdgpu.dc=0 which doesn't have this problem.