Most ATI GPUs, e.g. RX Vega 64, RX Vega Frontier, RX 580, RX 560 are supposed to be "natively" implemented. It is commonly recommended not to use RX 560 and RX 580 GPUs due to the lacking iGPU implementation when using SMBIOS iMacPro1,1.
By means of my Vega SSDT PCI device implementation outlined in Section 9.2.2, all available ports on the Gigabyte Vega 64 (3x HDMI, 3x DP), including multi-monitor support and display hot plug are automatically and successfully implemented. However, it seems that DP-port issues (black screen) remain, when employing the respective Vega SSDT PCI device implementation on Sapphire Nitro+ Vega 64 (2x HDMI, 2xDP) or ASUS Strix Vega 64 (2x HDMI, 2x DP, 1x DVI) GPUs, which only can be removed by adding the actual version of Whatevergreen.kext. Within such configuration, multi-monitor support and display hot plug can be fully achieved also in case of the latter custom Vegas (both 10.13.6 SU and 10.14.1). A fall-back to the formerly used Kamarang frame buffer SSDT implementation of @apfelnico does not provide any valuable solution for these kind of custom Vegas. Also note that Whatevergreen.kext only seems required for all Vegas possessing less than 6 physical ports (including the original Vegas like Frontier and Vega 64), else the current Vega SSDT PCI device implementation does pretty well it's job including fully successful system sleep/wake..