drm/tegra: rgb: Parameterize V- and H-sync polarities

The polarities of the V- and H-sync signals are encoded as flags in the
display mode, so use the existing information to setup the signals for
the RGB interface.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
[tomi.valkeinen@ideasonboard.com: default to positive sync]
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-v4-1-31725f008a50@ideasonboard.com