I have met the same problem as you! But mine is ubuntu16.04.7. I try for a long time, but still not solve it. Have you solved it now?
My problem’s details is in this link:
https://forums.developer.nvidia.com/t/ubuntu-16-04-installs-nvidia-driver-llvmpipe-llvm-6-0-256bits/78100/3?u=931476138