drm/i915: Maintain consistent documentation subsection ordering

With Sphinx 2.0 (or prior versions with the deprecation warnings fixed) the
docs build fails with:

  Documentation/gpu/i915.rst:403: WARNING: Title level inconsistent:

  Global GTT Fence Handling
  ~~~~~~~~~~~~~~~~~~~~~~~~~

  reST markup error:
  Documentation/gpu/i915.rst:403: (SEVERE/4) Title level inconsistent:

I "fixed" it by changing the subsections in i915.rst, but that didn't seem
like the correct change.  It turns out that a couple of i915 files create
their own subsections in kerneldoc comments using apostrophes as the
heading marker:

  Layout
  ''''''

That breaks the normal subsection marker ordering, and newer Sphinx is
rather more strict about enforcing that ordering.  So fix the offending
comments to make Sphinx happy.

(This is unfortunate, in that kerneldoc comments shouldn't need to be aware
of where they might be included in the heading hierarchy, but I don't see
a better way around it).

Cc: stable@vger.kernel.org  # v4.14+
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>