The only way I could get the xf86-video-odroid-c1 driver to work correctly was to use a display manager (lxdm).
I wiped the system, reinstalled, created a non-privileged user, then installed these packages in this order:
- xf86-video-odroid-c1
- xorg-server xorg-server-utils xorg-xinit
- xfce4
- lxdm
I rebooted, then edited the lxdm.conf file to make sure it launched xfce, enabled/started lxdm (via systemctl) and everything worked great after that.
Note that I did not place an .xinitrc file in the non-privileged user folder or do any other X-related config.