drm/bridge: display-connector: add ddc-en gpio support

"hdmi-connector.yaml" bindings defines an optional property
"ddc-en-gpios" for a single gpio to enable DDC operation.

Usually this controls +5V power on the HDMI connector.
This +5V may also be needed for HPD.

This was not reflected in code but is needed to make the CI20
board work.

Now, the driver activates the ddc gpio after probe and
deactivates after remove so it is "almost on".

But only if this driver is loaded (and not e.g. blacklisted
as module).

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/3607e924b7c0cf0be956c0d49894be1442dbda41.1649330171.git.hns@goldelico.com