drm: qxl: Expose creation of universal primary plane

Let's expose the primary plane initialization inside the qxl driver in
preparation for universal planes and atomic.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227204328.18761-5-krisman@collabora.co.uk
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>