Should be supported although there has been driver updates and some back and forth changes (radeon vs radeonhd) in 5.1 current.
Any clues in /var/log/Xorg.0.log? If you substitute
for
in your xorg.conf
Code:
Identifier "Card0"
Driver "radeonhd"
BusID "PCI:1:0:0"
does it help?