drm/doc: move composition function docs to new section

Move drm_blend.c 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/20201216202222.48146-3-contact@emersion.fr