Direct Rendering Manager has been a part of the kernel since 1999, before the 2.4 release. At that time the term Digital Rights Management as a catch-all for CSS, dongles, etc wasn't widespread.
I don't doubt it, but it was still a little frustrating -- as far as I can tell, DRM is never defined in the documentation on kernel.org. (I just doublechecked that by searching for "direct rendering" and nothing like "DRM (Direct Rendering Manager)" comes up.) I could tell from context that this didn't mean "digital rights management," but as someone who is not a GPU developer, I didn't have the context to figure it out.
So, I really think they should take a few extra words and put this in either the introduction to the GPU Driver Developer's Guide or the first page in the section entitled "DRM Internals". They do that for KMS!