older GCN1 cards may not inicialize AMDGPU drvers. 
In my experience, AMDgpu drivers may be more CPU intensive. 
But the AMDGPU driver gives more FPS in GPU bound situations. 

edit Grub kernel parameters
    sudo nano /etc/default/grub
or in grub-custimizer app


on the line with GRUB_CMDLINE_LINUX_DEFAULT=„loglevel=4“ add this 

    amdgpu.si_support=1 radeon.si_support=0

EG:

    GRUB_CMDLINE_LINUX_DEFAULT="loglevel=4 amdgpu.si_support=1 radeon.si_support=0"

Or in Grub customizer -> general settings -> Kernel parameters and add amdgpu.si_support=1 radeon.si_support=0 to the box -> save

Update greb

    sudo update-grub

reboot