drm/doc: Drop :c:func: markup

Kernel sphinx has learned how to do that in

commit d74b0d31


Author: Jonathan Corbet <corbet@lwn.net>
Date:   Thu Apr 25 07:55:07 2019 -0600

    Docs: An initial automarkup extension for sphinx

Unfortunately it hasn't learned that yet for structures, so we're
stuck with the :c:type: noise for now still.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Cc:  Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204101933.861169-1-daniel.vetter@ffwll.ch