Although Debian 10 and other recent Linux include a amdgpu display driver, it is still very new and only a portion of the cpu range is fully supported.
If in any doubt, and you don't fancy wrestling with xorg.conf, then you can install the non-free firmware 'firmware-amd-graphics' (see link)
This was the quickest solution for me and got me up and running with a Graphical desktop.
Note: Ensure you have non-free enabled in sources.list or manually download the deb to install.
Links and ref:
