OMAPDSS: DSI: Maintain own copy of timings in driver data

The DSI driver currently relies on the timings in omap_dss_device struct to
configure the DISPC and DSI blocks accordingly. This makes the DSI interface
driver dependent on the omap_dss_device struct.

Make the DSI driver data maintain it's own timings field. A DSI video mode panel
driver is expected to call omapdss_dsi_set_timings() to set these timings before
the panel is enabled.

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