drm/bridge: sii902x: add audio graph card support

Implement get_dai_id callback of audio HDMI codec
to support ASoC audio graph card.
HDMI audio output has to be connected to sii902x port 3.
get_dai_id callback maps this port to ASoC DAI index 0.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1562141052-26221-1-git-send-email-olivier.moysan@st.com