drm/qxl: unpin release objects

Balances the qxl_create_bo(..., pinned=true, ...);
call in qxl_release_bo_alloc().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210204145712.1531203-5-kraxel@redhat.com