drm/doc: use proper cross-references for sections

When I originally wrote these docs, I couldn't manage to insert a
cross-reference to a section. Here's how it can be done.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230803095734.386761-1-contact@emersion.fr