According to https://en.wikipedia.org/wiki/List_of_AMD_graphics_processing_units
your card is Terascale and not GCN. You just missed the cutoff unfortunately. That means your card uses the 'Radeon' instead of 'AMDGPU' driver. There is no Vulkan driver for pre-GCN cards.
The previous reply about drivers is incorrect to my knowledge, as the Radeon kernel driver is part of Mesa and I didn't hear anything about it being deprecated. It doesn't surprise me there is a Catalyst driver available also, but back when I heard only bad things about it. Also if you look at the release dates on the Catalyst drivers... yeesh.
I don't know how to make KDE use your card for primary output (I don't use KDE) but I would be surprised if switching drivers was the answer. I would also be surprised if the Catalyst driver worked at all with other OS components ten years newer. 🙂
Good luck!
