drm/qxl: move extern variable declaration header file

Flagged by sparse.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620113916.6967-4-kraxel@redhat.com