drm/qxl: handle shadow in primary destroy

qxl_primary_atomic_disable must check whenever the framebuffer bo has a
shadow surface and in case it has check the shadow primary status.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210204145712.1531203-8-kraxel@redhat.com