drm/doc: use cross-references for macros

These will show up as monospace, and will get linkified as soon as
we document the macro they point to.

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230712135723.173506-1-contact@emersion.fr