drm/bridge: lvds-encoder: add powerdown-gpios support

Optionally power down the LVDS-encoder when it is not in use.

Signed-off-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111151843.11114-6-peda@axentia.se