doc-rst: add stand-alone conf.py to gpu folder

With the gpu/conf.py, the gpu folder can be build and distributed
stand-alone. To compile only the html of 'gpu' folder use::

  make SPHINXDIRS="gpu" htmldocs
Signed-off-by: NMarkus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/gpu/conf.py

+# -*- coding: utf-8; mode: python -*-
+
+project = "Linux GPU Driver Developer's Guide"