OMAPDSS: remove extra EXPORT_SYMBOLs

The functions dispc_ovl_set_fifo_threshold and
dispc_ovl_compute_fifo_thresholds are exported, but not declared in
public headers, and thus are not used outside omapdss. So we can remove
the EXPORT_SYMBOL()s.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>