drm/bridge: tc358768: Default to positive h/v syncs

As the TC358768 is a DPI to DSI bridge, the DSI side does not need to
define h/v sync polarities. This means that sometimes we have a mode
without defined sync polarities, which does not work on the DPI side.

Add a mode_fixup hook to default to positive sync polarities.

Reviewed-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Tested-by: Maxim Schwalm <maxim.schwalm@gmail.com> # Asus TF700T
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230906-tc358768-v4-3-31725f008a50@ideasonboard.com