drm/bridge: lt9611: fix sleep mode setup

On atomic_post_disable the bridge goes to the low power state. However
the code disables too much of the chip, so the HPD event is not being
detected and delivered to the host. Reduce the power saving in order to
get the HPD event.

Fixes: 23278bf5 ("drm/bridge: Introduce LT9611 DSI to HDMI bridge")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>