As far as I know, MST is broken on the kernel layer for 5.1+ kernels bug. It's quite weird that it's working on your setup.
In userspace we have TILE property in xrandr and this bug and this
First one is already fixed(at least in 5.3.0-rc3)
For Cinnamon, if gtk use xrandr monitors feature, than just make sure you have xf86-video-amdgpu-19.0.1 installed ...