drm/sun4i: mixer: Read id from DT

Currently, TCON supports 2 ways to match TCON with engine (mixer in this
case). Old way is to just traverse of graph backwards and compare node
pointer. New way is to match TCON and engine by their respective ids.
All SoCs with DE2 enabled till now used the old way, which means mixer
id was never used and thus never implemented.

However, for R40, only the new way will be used. To prepare for that,
implement mixer id fetching from DT.

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/20180711112706.30222-1-jernej.skrabec@siol.net