drm/bridge: ti-tfp410: Add support for the powerdown GPIO

The TFP410 has a powerdown pin that can be connected to a GPIO to
control power saving. The DT bindings define a corresponding property,
but the driver doesn't implement support for it. Fix that.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>