OMAPDSS: add set_min_bus_tput pointer to omapdss's platform data

omapdss driver needs to use the omap_pm_set_min_bus_tput(), so add a new
entry for that in omapdss's platform data, and set it.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Acked-by: Kevin Hilman <khilman@ti.com>