Our update function is hooked to the single plane, which might not get
called for crtc-only updates. Which is surprising, so fix this by
always adding the plane.

While at it document how&when the event should be sent out better in
the kerneldoc.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: andrea.merello@gmail.com
Tested-and-Reported-by: andrea.merello@gmail.com
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Reviewed-by: NNoralf Trønnes <noralf@tronnes.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1471933540-31131-1-git-send-email-daniel.vetter@ffwll.ch