Whatevergreen is not necessary. If you don't use Whatevergreen you of course have to add the patches this kext does automatically to your config. In a nutshell you need the AGDP board-id patch and PEGP to GFX0 rename. I also recommend to disable the IGPU and use the iMacPro1,1 SMBIOS when using Polaris, Navi10, Navi14, Vega10 and Vega20 cards. This gives you working DRM for Netflix in Safari and enables video encoding / decoding through the AMD card.