drm: qxl: Atomic phase 3: Use atomic handlers for planes

Now that the state objects are wired up, we can move to the final atomic
handlers.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227204328.18761-12-krisman@collabora.co.uk


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>