OMAPDSS: remove uses of dss_runtime_get/put

Now that the omapdss_core device is the parent for all other dss
devices, we don't need to use the dss_runtime_get/put anymore. Instead,
enabling omapdss_core will happen automatically when a child device is
enabled.

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