Documentation/gpu: add new gpu.rst converted from DocBook gpu.tmpl

This is the first step towards converting the DocBook gpu.tmpl to Sphinx
and reStructuredText, the new kernel documentation tool and markup.

Use Jon's "cheesy conversion script" in Documentation/sphinx/tmplcvt to
do the rough conversion. Do the manual edits in follow-up patches. Add a
new Documentation/gpu directories for the graphics related
documentation. (Hooray, now we can have directories based on topics
rather than tools under Documentation.)

We also won't remove the DocBook gpu.tmpl yet so it's easier to build
both and compare the results for parity.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/bc7b4f9ac037632e0c8469c079d21fad5eaa39a0.1466506505.git.jani.nikula@intel.com