drm/sun4i: Tie the DSI controller in the TCON

The DSI controller needs a particular interface (CPU aka 8080) with some
modifications from the TCON in order to run.

Make sure the TCON is able to provide it when we are using the DSI output.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/129f5928113d2ca865bf5269047c2e4ba6fed5e6.1522835818.git-series.maxime.ripard@bootlin.com