drm/bridge: anx7625: send DPCD command to downstream

Send DPCD command to downstream before anx7625 power down,
let downstream monitor enter into standby mode.

Signed-off-by: Xin Ji <xji@analogixsemi.com>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220213103437.3363848-1-hsinyi@chromium.org