drm/bridge: dw-hdmi-i2s: add .get_eld support

Provide the eld to the generic hdmi-codec driver.
This will let the driver enforce the maximum channel number and set the
channel allocation depending on the hdmi sink.

Cc: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812125016.20169-1-jbrunet@baylibre.com