drm/ingenic: Add dw-hdmi driver specialization for jz4780

A specialisation of the generic Synopsys HDMI driver is employed for
JZ4780 HDMI support. This requires a new driver, plus device tree and
configuration modifications.

Here we add Kconfig DRM_INGENIC_DW_HDMI, Makefile and driver code.

Note that there is no hpd-gpio installed on the CI20 board HDMI
connector. Hence there is no hpd detection by the connector driver
and we have to enable polling in the dw-hdmi core driver.

For that we need to set .poll_enabled but that struct component
can only be accessed by core code. Hence we use the public
setter function drm_kms_helper_hotplug_event() introduced before.

Also note that we disable Color Space Conversion since it is not
working on jz4780.

Signed-off-by: Paul Boddie <paul@boddie.org.uk>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/e5cdf9cd44bde52cce379cc830f2d6117ea15c32.1649330171.git.hns@goldelico.com