Problem : "I can't start Xorg system because he don't find my first screen (Radeon GPU)"

Solve :
- Run "startx -configure"
- Copy the new Xorg config in /etc/X11/xorg.conf
- Edit this config : Driver "vesa"
- Run touch driver-radeon.conf in /usr/local/etc/X11/xorg.conf.d/
- Edit this config and add :
| Section "Device"
|         Identifier "Card0"
|         Driver "vesa"
| EndSection
(Infos : https://www.freebsd.org/doc/handbook/x-config.html)