OMAPDSS: DISPC: make _enable_mgr_out public as "dispc_mgr_enable"

We need a low level manager-enable function for omapdrm. We have that
function as dispc internal func, _enable_mgr_out().

This patch exposes that function, and renames it to dispc_mgr_enable().

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