Gfxinfo: Add NVIDIA GPU support

Since neither Nouveau nor envytools use PCIIDs to identify GPUs, the best source of information I found is https://download.nvidia.com/XFree86/Linux-x86_64/535.54.03/README/supportedchips.html (with https://download.nvidia.com/XFree86/Linux-x86_64/latest.txt to know which one is the latest version).

Parsing will be a pain, but at least it seems to show all NVIDIA GPUs supported by the proprietary driver.