drm/sun4i: tcon: Check for multiple paths between TCONs and backends

The patch b317fa3b ("drm/sun4i: tcon: Find matching display backend
by device node matching") assumed a one-to-one mapping between TCONs
and backends. This turned out wrong, as we found muxing controls in the
TCON of the A31, and undocumented usage of the backend output selector
of the A20.

Make sun4i_tcon_find_engine() bail out if the current node has multiple
input connections.

Fixes: b317fa3b

 ("drm/sun4i: tcon: Find matching display backend
		      by device node matching")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908075016.18657-4-wens@csie.org