drm/sun4i: Add support for traversing graph with TCON TOP

TCON TOP is different from other nodes in graph by having 3 input and 3
output ports. Additionally, connection to TV TCON might lead back to
HDMI mux input port, creating loops.

Add support for traversing such graph.

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/20180625120304.7543-9-jernej.skrabec@siol.net