OMAPDSS: VENC: Maintain copy of venc type in driver data

The VENC driver currently relies on the omap_dss_device struct to configure the
venc type. This makes the VENC interface driver dependent on the omap_dss_device
struct.

Make the VENC driver data maintain it's own 'venc type' field. A panel driver
is expected to call omapdss_venc_set_type() before enabling the interface or
changing the type via display sysfs attributes.

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