drm/bridge: tc358767: add bus flags

tc358767 driver does not set DRM bus_flags, even if it does configures
the polarity settings into its registers. This means that the DPI source
can't configure the polarities correctly.

Add sync flags accordingly.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190103115954.12785-2-tomi.valkeinen@ti.com