I have a RX480 card which has the sleep problem in 10.13.2 and I chose to go down the "edit plist" route, as it seems more straightforward and more importantly I don't have windows.
Here is my process, incase it helps anyone. There is probably a simpler way of doing it, if so maybe someone can enlighten me.
By following the advice give in various threads...
- Make sure SIP is disabled and download a copy of Kext Utility
- find this kext: /System/Library/Extensions/AMD9500Controller.kext and make a duplicate of it in case you need to replace it
- find this file: /System/Library/Extensions/AMD9500Controller.kext/Contents/Info.plist
- copy Info.plist to your desktop and then open this copied file in Text Edit
- search for this text :<key>CFG_FB_LIMIT</key> <integer>0</integer>
- change it to :<key>CFG_FB_LIMIT</key> <integer>4</integer> The number 4 is the number of ports on the back of the card. I have the reference 480 card if you have a Nitro 480 then the number should be 5 because the Nitro has an extra DVI port on the back.
- save the edited Info.plist file to your desktop
- go to /System/Library/Extensions/AMD9500Controller.kext/Contents/ and delete the original info.plist file
- copy the new version from the desktop to replace the deleted file.
- run Kext Utility to repair permissions
restart and enjoy!
Also this fixes audio over Displayport.
This seems to work for RX 4xx/5xx cards, in another thread someone tried to do a similar process to fix sleep on their RX Vega 56 but it just ended up with a black screen.
edit 25 jan 2018:
Installed mac OS 10.13.3 from App Store and my edited Kext survived the update. SIP remains disabled.