I switched to Ubuntu 17.10 from 17.04, where the same xorg.conf worked in /etc/xorg.conf, and Ubuntu seems to ignore the file. My workaround is the same as before in Ubuntu 17.04, I copied xorg.conf to /etc. But it wasn't loaded. I use the xorg.conf to set the right resolution and the driver. The xorg.conf would load the radeon driver and
lshw -c video said that this driver is already loaded. I tried other locations for xorg.conf like the manpage sugguest without luck.
I hope someone of you can help me.