drm/sun4i: tcon: get TCON ID and matching engine with remote endpoint ID

The device tree binding for sun4i-drm says:

    For all connections between components up to the TCONs in the display
    pipeline, when there are multiple components of the same type at the
    same depth, the local endpoint ID must be the same as the remote
    component's index. For example, if the remote endpoint is Frontend 1,
    then the local endpoint ID must be 1.

We should be able to get the TCON's ID directly from any of the remote
endpoints from its input port. With the ID, we can then go through the
list of registered engines and find a matching one by ID.

However the A31 device tree is incorrect. We assumed that there were no
cross pipeline connections between the backends and TCONs. As a result,
in all the endpoints between the backends and TCONs of the second
display pipeline, the endpoint IDs were incorrectly set to 0, when in
fact they should've been set to 1.

To maintain compatibility with this incorrect device tree, we first
check if the TCON's input port has 1 or many endpoints. If there are
more than 1, then it is likely a fixed version, and we can proceed
with the new method. If there is only 1 endpoint, then it is possibly
an incorrect version, or it could be the SoC only has one pipeline.
In either case we fall back to using the old method of traversing
the input connections to find a matching engine, and then get its
ID.

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/20170908075016.18657-5-wens@csie.org