OMAPDSS: RFBI: Use dss_mgr_enable to enable the overlay manager

The RFBI driver uses a direct DISPC register write to enable the overlay
manager. Replace this with dss_mgr_enable() which checks if the connected
overlay and managers are correctly configured, and configure DSS for
fifomerge.

Signed-off-by: Archit Taneja <archit@ti.com>