drm/sun4i: tcon: Copy ID from associated backend

The tcons and backends have a one-to-one relationship. Their IDs,
or indexes in the documentation, are also the same.

Copy the ID from the associated backend and save it in the tcon
structure. This will later be used when we add support for the
output data path muxes.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>