drm: dw-hdmi-i2s: add .get_dai_id callback for ALSA SoC

ALSA SoC needs to know connected DAI ID for probing.
It is not a big problem if device/driver was only for sound,
but getting DAI ID will be difficult if device includes both
Video/Sound, like HDMI.
To solve this issue, this patch adds new .get_dai_id callback
on hdmi_codec_ops.
dw-hdmi-i2s will assume that HDMI sound will be connected
to reg = <2>. Then, ALSA SoC side will recognized it as DAI 0

	ports {
		#address-cells = <1>;
		#size-cells = <0>;
		port@0 {
			reg = <0>;
			/* HDMI Video IN */
		};
		port@1 {
			reg = <1>;
			/* HDMI OUT */
		};
		port@2 {
			reg = <2>;
			/* HDMI Sound IN */
		};
	};

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>