drm/qxl: use to_qxl_bo macro

Instead of using container_of directly use to_qxl_bo macro.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450178476-26284-9-git-send-email-boris.brezillon@free-electrons.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>