drm/doc: move damage tracking functions to new section

Move drm_damage_helper function reference from the KMS properties
section to the plane abstraction section. This makes the KMS
properties section more readable for user-space developers.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201216202222.48146-4-contact@emersion.fr