Gain massive GPU performance for Southern Islands AMD GPUs on Linux by removing DPM quirks in the kernel

I recently found out that my r9 270x GPU was not hitting full clock speeds because it was being throttled by the kernel based on old bug reports on DPM stability. This was an easy fix on gentoo where recompiling the kernel is trivial, but on ubuntu and similar, rebuilding the kernel is not so straightforward, so here's a guide based on ubuntu 16.04 LTS. If you run gentoo or know what you're doing you can just skip to the relevant code.

On my r9 270x, this resulted in an overall improvement in frame-rates of up to 50%: Unigine Valley went from something like 30-40 fps average to 55-60 and osu! (running in wine) went from 0.33 ms (~3000fps) to 0.25ms (~4000fps).

Note that I'm no linux god and all this info was pieced together by googling and asking on forums (credits to the guys at phoronix for pointing me to the quirks code).

If you're using the proprietary amdgpu-pro driver, read until the end of the