OMAPDSS: Fix omap_dss_find_output_by_port_node() port refcount decrement

Fix omap_dss_find_output_by_port_node() port parameter refcount
decrementation. The only user of dss_of_port_get_parent_device()
function is omap_dss_find_output_by_port_node() and it assumes the
refcount of the port parameter is not decremented by the call.

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