drm/sun4i: tcon: Add another way for matching mixers with tcon

Till now, new way of matching engines with TCONs was reading their
respective ids and match them by those ids. However, with introduction
of TCON TOP, that might not be so straightforward anymore.
- there might be more TCONs that engines (mixers)
- TCON ids might have non-consecutive ids

Workaround that by matching mixer id with TCON index from TCON list.

For example, R40 has 2 mixers and 4 TCONs. Board designer can choose
2 outputs, which are connected to any of those 4 TCONs. As long as there
are only 2 TCONs enabled in DT, using index in list as alternative id,
will allow to match them with mixer 0 and 1.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
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/20180710203511.18454-13-jernej.skrabec@siol.net