drm/bridge: lt9611: fix sync polarity for DVI output

Attaching DVI sink to the lt9611 requires different setup. Fix the
register write to make the DVI displays sync onto the correct sync
pulse.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230118081658.2198520-10-dmitry.baryshkov@linaro.org