Install OpenCL from amdgpu-pro under Ubuntu non supported version

[wallie]

And if the admins feel that this explanation should be moved to the main thread

https://discuss.pixls.us/t/using-amd-open-source-and-the-amdgpu-pro-opencl-driver-for-image-processing

than please append it there and delete it here or let me know, than I will do it.
I just felt, that the other thread points more to rocm in the meantime