drm/doc: fix drm_plane_type docs

The docs for enum drm_plane_type mention legacy IOCTLs, however the
plane type is not tied to legacy IOCTLs, the drm_cursor.primary and
cursor fields are. Add a small paragraph to reference these.

Instead, document expectations for primary and cursor planes for
non-legacy userspace. Note that these docs are for driver developers,
not userspace developers, so internal kernel APIs are mentionned.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115110626.12233-1-contact@emersion.fr