doc: Cope with the deprecation of AutoReporter

AutoReporter is going away; recent versions of sphinx emit a warning like:

  Documentation/sphinx/kerneldoc.py:125:
      RemovedInSphinx20Warning: AutodocReporter is now deprecated.
      Use sphinx.util.docutils.switch_source_input() instead.

Make the switch.  But switch_source_input() only showed up in 1.7, so we
have to do ugly version checks to keep things working in older versions.

Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>