The core drivers are part of the kernel but there are still things you should install to have it fully functional.
Here is what I would recommend installing:
lib32-mesa
lib32-vulkan-radeon
libva-mesa-driver
mesa
mesa-utils
mesa-vdpau
vulkan-radeon
xf86-video-amdgpu
I would install xf86-video-amdgpu before installing the card if possible. It won’t hurt anything to install it with whatever your current GPU is. It just won’t be used. It gets installed by default so you may already have it.