drm: dw-hdmi: add specific I2S and AHB functions for stream handling

Currently, CTS+N is forced to zero as a workaround of the IP block for
i.MX platforms. This is requested in the datasheet of the corresponding
IP for AHB mode only. However, we have seen that it introduces glitches
or delays when playing a sound on HDMI for I2S mode. This proves that we
cannot keep the current functions for handling audio stream as-is if
these contain workaround that are specific to a mode.

This commit introduces two callbacks, one for each variant.
dw_hdmi_setup defines the right function depending on the detected
variant. Then, the exported functions dw_hdmi_audio_enable and
dw_hdmi_audio_disable calls the corresponding callbacks

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Romain Perier <romain.perier@collabora.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170414083113.4255-2-romain.perier@collabora.com