OMAPDSS: DSI: Maintain copy of operation mode in driver data

The DSI driver currently relies on the omap_dss_device struct to know the mode
of operation of the DSI protocol(command or video mode). This makes the DSI
interface driver dependent on the omap_dss_device struct.

Make the DSI driver data maintain it's own operation mode field. The panel
driver is expected to call omapdss_dsi_set_operation_mode() before the interface
is enabled.

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