drm/sun4i: tcon: Generalize engine search algorithm

Current "old" method to find engine worked pretty well for DE2. However,
it doesn't work when TCON TOP is between  mixer (engine) and TCON. TCON
TOP has multiple input ports, but current engine search algorithm
expects only one.

This can be fixed by first looking for output port id and selecting
matching input by subtracting 1 for the next round. This work even if
there is only one input and output.

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/20180625120304.7543-11-jernej.skrabec@siol.net