drm: of: always initialize panel in drm_of_find_panel_or_bridge()

The callers expect "panel" to be initialized, but that isn't true if we
return -ENODEV.  It causes bugs like:

    drivers/gpu/drm/tve200/tve200_drv.c:83 tve200_modeset_init()
    error: uninitialized symbol 'panel'.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170925103038.lvr5msjvekwczctn@mwanda

drivers/gpu/drm/drm_of.c

@@ -233,6 +233,8 @@ int drm_of_find_panel_or_bridge(const struct device_node *np,
 
 	if (!panel && !bridge)
 		return -EINVAL;
+	if (panel)
+		*panel = NULL;
 
 	remote = of_graph_get_remote_node(np, port, endpoint);
 	if (!remote)