Section "Device"
        Identifier "Radeon R9 285"
        Driver "amdgpu"
        Option "AccelMethod" "glamor"
        Option "DRI" "3"
        Option "TearFree" "on"
        Option "ColorTiling" "on"
        Option "ColorTiling2D" "on"
EndSection

This was all in 20-amdgpu.conf originally, doesn't matter if I add
monitors+screen or not. I have to run these commands after every restart, or no
signal:

xrandr --newmode "mymode" 148.35  1920 2008 2052 2200  1080 1084 1089 1125
+hsync +vsync
xrandr --addmode DVI-I-1 mymode
xrandr --output DVI-I-1 --right-of DVI-D-0 --mode mymode