docs: automarkup: do not look up symbols twice

The automarkup code tries to look up symbols once as a function, and once
as a macro.  The Sphinx C domain code, though, totally ignores that
distinction and will return the same results either way.  So just look
things up once and be done with it; the resulting output does not change,
but htmldocs build time drops by about 5%.

Tested-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/20220630163630.714673-3-corbet@lwn.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>