drm: qxl: Atomic phase 1: convert cursor to universal plane

In preparation for atomic conversion, let's use the transitional atomic
helpers drm_plane_helper_update/disable.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227204328.18761-6-krisman@collabora.co.uk
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>