Just prep work to polish and consolidate all the dma-buf related
documenation.

Unfortunately I didn't discover a way to both integrate this new file
into the overall toc while keeping it at the current place. Work
around that by moving it into the overall driver-api/index.rst.

Cc: linux-doc@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Few files under dma-buf/ changed their names but the changes didn't
applied to a document that referencing them.  It is causing few
documentation build warnings.  This commit fixes the problems by
applying changed file names on the document.
Signed-off-by: NSeongJae Park <sj38.park@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

We don't need to keep it as a single large file anymore; split it up so
that it is easier to manage and the individual sections can be read
directly as plain files.
Signed-off-by: NJonathan Corbet <corbet@lwn.net>