drm/meson: imply dw-hdmi i2s audio for meson hdmi

Imply the i2s part of the Synopsys HDMI driver for Amlogic SoCs.
This will enable the i2s part by default when meson hdmi driver
is enable but let platforms not supported by the audio subsystem
disable it if necessary.

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/20190429102325.29022-1-jbrunet@baylibre.com