drm/sun4i: Split out code for enumerating endpoints in output port

Until now, each node has one input port and one output port. However,
with TCON TOP this is no longer true. It has 3 input and 3 output ports.

In order to prepare to this situation, split out the code which checks
all endpoints in input port and adds available components to fifo.

This patch doesn't do any functional change.

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/20180625120304.7543-8-jernej.skrabec@siol.net