drm/sun4i: backend: Support output muxing

The backend has a mux to select the destination of the data to output
to. It can select the TCON or the frontends. On the A20, it includes
an option to output to the second TCON. This is not documented in the
user manual, but the vendor kernel uses it nevertheless, so the second
backend outputs to the second TCON.

Although the muxing can be changed on the fly, DRM needs to be able to
group a bunch of layers such that they get switched to another crtc
together. This is because the display backend does the layer compositing,
while the TCON generates the display timings. This constraint is not
supported by DRM.

Here we simply pair up backends and TCONs with the same 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/20171017121807.2994-2-wens@csie.org