Have you enabled Southern Islands support as a kernel parameter? Your generation of GPU was originally supported on radeon, so you need to explicitly enable SI (Southern Islands) support to use amdgpu.

See ArchWiki for more information

There is a patch-series that is now merged into 6.15-rc1 that exposes more i2c/RGB controllers on the GPU side for AMD cards, so that kernel + OpenRGB (as others have mentioned) might be a solution down the line

Patch series for reference: https://lists.freedesktop.org/archives/amd-gfx/2025-January/118399.html