If you don't have time to read my story (which also describes some interesting technical approaches) just scroll down this thread until a "100% WORKING SOLUTION" text
Discrete AMD GPU of my 2011 MacBook Pro 8,2 has finally failed because of the reasons mentioned here (...
Installing GRUB
Follow the steps described in this guide:
https://gist.github.com/blackgate/17ac402e35d2f7e0f1c9708db3dc7a44
Installing the attached kext to prevent the GPU from waking up from sleep (optional)
WARNING: This kext should only be used with the GRUB solution in place, otherwise...