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

drivers/gpu/drm/qxl/qxl_display.c

@@ -801,6 +801,7 @@ static void qxl_prepare_shadow(struct qxl_device *qdev, struct qxl_bo *user_bo,
 	    qdev->dumb_shadow_bo->surf.width  != surf.width ||
 	    qdev->dumb_shadow_bo->surf.height != surf.height) {
 		if (qdev->dumb_shadow_bo) {
+			qxl_bo_unpin(qdev->dumb_shadow_bo);
 			drm_gem_object_put
 				(&qdev->dumb_shadow_bo->tbo.base);
 			qdev->dumb_shadow_bo = NULL;