drm: better document how to send out the crtc disable event

The kernel doc explained what needs to happen, but not how to most
easily accomplish that using the functions. Fix that.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-14-daniel.vetter@ffwll.ch