I followed the recommendations of this page:
https://wiki.archlinux.org/index.php/Hybrid_graphics

The call working on my system is: echo "\\_SB.PCI0.PEG0.PEGP._OFF" >
/proc/acpi/call

If I use this command on a system that does not have amdgpu blacklisted,
however, this will lead to unstable system behaviour and a certain crash within
~5 min.