I have the same gpu and solved the problem! You don't need any other kexts except for the one that come with the niresh installation (so don't just go deleting some and adding some).
1.) Locate AMD7000Controller.kext and AMDRadeonX4000.kext/ in S / L / E
2.) Drag these two files on to the desktop
3.) Right click on AMD7000Controller.kext and choose "Show Package Contents"
4.) Now inside the kext, go into "Contents" then open Info.plist
5.) Inside Info.plist, Press command & f together to pull up a search box
6.) Search for the number sequence "0x68181002" and replace the second number "8" with a number "9"
7.) This should be the new number: "0x68191002". Save that Info.plist and install the kext with Kext Utility
8.) Now do the exact same process for the other kext file: AMDRadeonX4000.kext
Now that you edited and installed those two kexts, you need the OS to recognize your Graphics Card on boot
1.) Locate /Extra folder that is in your Mac OS drive and look for Boot.plist (may be called something else but it always has "boot" in it)
2.) Add this boot option to your boot plist:<key>device-properties</key>
Note: Those numbers are the hexadecimal code for my Graphics Card (AMD Sapphire HD 7850 2GB), so assuming we have the same GPU, this code will work.
If you need a different code for a different GPU, that will a series of extra steps
If all goes well, your Hackintosh should fully utilize and recognize your Graphics Card. Enjoy the smooth graphics!!!