drm/sun4i: tcon: Release node when traversing of graph

Function sun4i_tcon_find_engine_traverse() doesn't release node if it
needs to traverse of graph deeper than 1 level.

Fix this by calling of_node_put().

Fixes: 49836b11 ("drm/sun4i: tcon: Generalize engine search algorithm")

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180710203511.18454-6-jernej.skrabec@siol.net