drm: qxl: Atomic phase 1: Use drm_plane_helpers for primary plane

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227204328.18761-7-krisman@collabora.co.uk


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