Documentation/gpu: use recommended order of heading markers

While splitting the document up, the headings "shifted" from what pandoc
generated. Use the following order for headings for consistency:

==============
Document title
==============

First
=====

Second
------

Third
~~~~~

Leave the lower level headings as they are; I think those are less
important. Although RST doesn't mandate a specific order ("Rather than
imposing a fixed number and order of section title adornment styles, the
order enforced will be the order as encountered."), having the higher
levels the same overall makes it easier to follow the documents.

[I'm sort of kind of writing the recommendation for docs-next in the
mean time, but this order seems sensible, and is what I'm proposing.]

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/126f42734defac6cbb8496a481d58db7b38461dd.1466506505.git.jani.nikula@intel.com