drm/sun4i: Set drm_crtc.port to the underlying TCON's output port node

The way drm_of_find_possible_crtcs works is it tries to match the
remote-endpoint of the given node's various endpoints to all the
crtc's .port field. Thus we need to set drm_crtc.port to the output
port node of the underlying TCON.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>