Documentation/sphinx: make targets independent of Sphinx work for HAVE_SPHINX=0

Make targets that don't depend on Sphinx work without warnings about
missing Sphinx. 'make cleandocs' will work without Sphinx just fine, and
the targets that are no-ops for Sphinx should just be skipped. Move them
outside of the HAVE_SPHINX checks to take precedence over the .DEFAULT
target for HAVE_SPHINX=0.

Reported-by: Jim Davis <jim.epost@gmail.com>
Reference: http://lkml.kernel.org/r/CA+r1ZhjRVqkjPXGOGB_BOAX2Hkfb+qQCtTzFfBMFeH1Mfeej7w@mail.gmail.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>