drm/bridge: dw-hdmi: move audio channel setup out of ahb

Part of the channel count setup done in dw-hdmi ahb should
actually be done whatever the interface providing the data.

Let's move it to dw-hdmi driver instead.

Reviewed-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812120726.1528-3-jbrunet@baylibre.com