OMAPDSS: DSI: Rename dsi_videomode_data to dsi_videomode_timings

The struct omap_dss_dsi_videomode_data holds fields which need to be configured
for DSI to operate in video mode. Rename the struct to dsi_videomode_timings.

One reason to do this is because most of the fields in the struct are timings
related. The other reason is to create a generic op for output specific
timings. This generic op can be considered as a way to set custom or private
timings for the output.

In the case of OMAP, DSI and RFBI require some more timings apart from the
relgular DISPC timings. The structs omap_dss_videomode_timings and rfbi_timings
can be considered as these output specific timings respectively.

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