That's what happens when using the Radeon driver without the proprietary
firmware blobs doesn't work, if I'm not mistaken. It falls back to a generic
driver that can only display up to that resolution.
Ideally, you should replace the Radeon GPU with one that is properly
supported by libre software; Radeon GPUs are about the worst ones you can
get. If your setup is an Intel CPU with a Radeon card, you can just pull the
Radeon card out and use the Intel integrated GPU; this would be the best
solution.
Otherwise, you'll have to get an Nvidia card that is supported by Nouveau.
The best candidates are ones a couple years old, but this is just a general
rule, so do some research. If you have a PCI Express slot, you can just get
the GeForce card sold by Think Penguin, under "Accessories & Other Products":
http://libre.thinkpenguin.com
If you use that link to get there, a portion of the profits will be donated
to Trisquel.
