drm/bridge: it6505: Send DPCD SET_POWER to downstream

Send DPCD SET_POWER command to downstream in .atomic_disable to make the
downstream monitor enter the power down mode, so the device suspend won't
be affected.

Fixes: b5c84a9e

 ("drm/bridge: add it6505 driver")
Signed-off-by: Pin-Yen Lin <treapking@chromium.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220425134424.1150965-1-treapking@chromium.org
Reviewed-by: Robert Foss <robert.foss@linaro.org>