In this line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
Make it look exactly like:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash amdgpu.si_support=1 radeon.si_support=0
There is one space between each parameter.
You can see above that you go from two parameters quiet and splash to Four parameters.
Tap ctl+o to overwrite, then enter to save as current configuration.
Tap ctl+x to exit the editor.
Now run
sudo update-grub
Reboot and test.