Yeah I installed high sierra last night and using the R9 280x it worked perfect but after it installs and reboots to get to the setting up page, like keyboard, language and user name etc.... it goes black.
I'll try using my IGPU first and see if it lets me in when I get home from work.
Are you installing macOS? If so, just use IGPU first for install, then you can try to boot with your AMD first when macOS is installed. For my desktop with AMD R9 280X, it works without any kind of kext except cosmetics detail with ssdt patch to show proper ROM + EFI + the model.
So just boot up with IGPU as primary when you try to install the macOS, then after install finish, you can boot with discrete graphic as first!