qxl: fix framebuffer unpinning

qxl_plane_cleanup_fb() unpins the just activated framebuffer
instead of the old one.  Oops.  Fix it.

Cc: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Fixes: 1277eed5fecb8830c8cc414ad70c1ef640464bc0
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170918074145.2257-1-kraxel@redhat.com

drivers/gpu/drm/qxl/qxl_display.c

@@ -702,14 +702,15 @@ static void qxl_plane_cleanup_fb(struct drm_plane *plane,
 	struct drm_gem_object *obj;
 	struct qxl_bo *user_bo;
 
-	if (!plane->state->fb) {
-		/* we never executed prepare_fb, so there's nothing to
+	if (!old_state->fb) {
+		/*
+		 * we never executed prepare_fb, so there's nothing to
 		 * unpin.
 		 */
 		return;
 	}
 
-	obj = to_qxl_framebuffer(plane->state->fb)->obj;
+	obj = to_qxl_framebuffer(old_state->fb)->obj;
 	user_bo = gem_to_qxl_bo(obj);
 	qxl_bo_unpin(user_bo);
 }