drm/qxl: balance dumb_shadow_bo pin

The shadow bo is created in pinned state, so we have to unpin it when
dropping the reference.  Otherwise ttm is unhappy and throws a WARN()
on release.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210511104522.2694803-3-kraxel@redhat.com