OMAPDSS: DSI: improve DSI module id handling

We currently use the id of the dsi platform device (dsidev->id) as the
DSI hardware module ID. This works because we assign the ID manually in
arch/arm/mach-omap2/display.c at boot time.

However, with device tree the platform device IDs are automatically
assigned to an arbitrary number, and we can't use it.

Instead of using dsidev->id during operation, this patch stores the
value of dsidev->id to a private field of the dsi driver at probe(). The
future device tree code can thus set the private field with some other
way.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>