well it doesnt work with just ATI Inject ... u will have to add your device/vendor id in the IOPCIMatch Part of the Juniper Graphics Accelerator in the Info.plist inside the AMDRadeonX3000.kext and also in the AMD5000Controller.kext
(right click kext-> Show Package Contents)( this method Works since 10.7-10.12.1 used it myself like that for years)
after u edited them on the desktop reinstall them using Kext Utility set your ATI Inject in Clover config.plist to false ! reboot and enjoy Full Graphics.
(it will assign the default Radeon Framebuffer which sometimes isnt the rite one for some people ) if this is the case u can set ATI Config= Vervet in Clover to have slightly better performance)
also if you do it this way u would have to do it over again with the next update ... Binarie or Info.plist KextPatch via Clover would be an alternative for that .