OpenCL support for Vega 64 through amdgpu-pro 17.40

[alexanderkjeldaas]

Issue description

There's a amdgpu-pro 17.40 mining edition that should be supported by nixpkgs.
I have one commit in https://github.com/alexanderkjeldaas/nixpkgs/tree/ak/amdgpu-pro-17.40 for this.

Getting this to compile might require the ROCK kernel at https://github.com/RadeonOpenCompute/ROCK-Kernel-Driver or it might require patches. I'm not sure, but the ROCK kernel driver might have kfd changes that are not upstream.

Some more info:
https://www.phoronix.com/scan.php?page=article&item=radeon-vega-setup&num=1
(the firmware updates mentioned should be in linux-firmware now).

[alexanderkjeldaas]

nix-build -A amdgpu-pro works in my branch now.