You shoudn't try add ubuntu repositories to a debian system.
Also, hw accelerated mesa requires you to update dri/drm libraries and associated kernel modules.That libdrm_radeon might just be the beginning. By manually updating all that, you will end up having a lot of non-distro stuff. And it won't be fun to compile and update all that (not even mentioning dist-upgrades).
As an alternative, you could try to install the fglrx proprietary driver (for your GPU, you will need the "legacy" version). Have a look at this debian wiki page for the recommended way to install this on debian.