Black screen right before login after upgrading to RX 580

[Gigamaxx]

This got me to finally see the login screen for the first time but I have to keep IGFX enabled and as the main display in my bios settings. This means I have to keep multimonitor on in mac when im only using one monitor. How can I make the system think that my GPU is the IGFX?

You might try Mac Pro 5.1 or 6.1 definition.