OMAPDSS: VENC: Maintian copy of video output polarity info in private data

The VENC driver currently relies on the omap_dss_device struct to configure the
video output polarity. This makes the VENC interface driver dependent on the
omap_dss_device struct.

Make the VENC driver data maintain it's own polarity field. A panel driver
is expected to call omapdss_venc_invert_vid_out_polarity() before enabling the
interface.

Signed-off-by: Archit Taneja <archit@ti.com>