OMAPDSS: DSI: Maintain copy of video mode timings in driver data

The DSI driver currently relies on the omap_dss_device struct to receive the
video mode timings requested by the panel driver. This makes the DSI interface
driver dependent on the omap_dss_device struct.

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

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