OMAP: DSS2: Change DSI device naming

Currently, there are 2 differently named platform devices generated for
the 2 DSS DSI modules. In order to use the same driver, the dsi devices
should be 2 instances of the same platform device.

Change the platform device names from "omapdss_dsi1" and "omapdss_dsi2"
to omapdss_dsi", and set the device indices to 0 and 1.

Signed-off-by: Archit Taneja <archit@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>