drm: document DRM_IOCTL_MODE_RMFB

Since there's no struct to attach the docs to, document the IOCTL
definition.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ephVkof3uGu2RpOdBbrHE3qF98zBfIBRzXe4Vyoboh0@cp4-web-034.plabs.ch