OMAPDSS: DSI: remove dsi_get_num_lanes_used

There's no longer need for the dsi_get_num_lanes_used function, so it
can be removed. The lane check in dsi_init_display() can be removed as
the validity of the config will be verified when parsing it.

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