OMAPDSS: Add DT support to VENC

Add DT support to VENC.

In contrast to non-DT version, the DT version gets the invert-polarity
and connector type via venc's endpoint, not from the connector.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>