Radeon kernel modesetting for r600 or later requires firmware-linux-nonfree

Radeon kernel modesetting for r600 or later requires firmware-linux-nonfree

nano /etc/apt/sources.list

deb http://ftp.pt.debian.org/debian stretch main
# change from above to bellow
deb http://ftp.pt.debian.org/debian stretch main non-free

apt-get update
apt-get install firmware-linux-nonfree