drm: document drm_mode_create_lease object requirements

validate_lease expects one CRTC, one connector and one plane.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Keith Packard <keithp@keithp.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210903130000.1590-1-contact@emersion.fr