Do you have
deb
http://ftp.uk.debian.org/debian unstable main non-free
within your /etc/apt/sources.list.d/debian.list
Also you get around having to edit the new and complex grub2 configuration you can as a permanent insert place
options radeon.modeset=0
into
/etc/modprobe.d/radeon.conf (if you have that).
If you are able to install the linux-firmware-nonfree package then you may have in /etc/modprobe.d/radeon-kms.conf. Just place the line option radeon modeset=0 in there instead of /etc/modprobe.d/radeon.conf.
Kms will be permanently disabled this way.
And while you are about it if you have pcie card as opposed to agp you can place "options radeon agpmode=-1" into /etc/modprobe.d/radeon-kms.conf or /etc/modprobe.d/radeon.conf (depending on what you have).