OMAPDSS: split overlay manager creation

Split the function that creates overlay manager structs into two: one
that creates just the structs, and one that creates the sysfs files for
the manager.

This will help us use the overlay manager structs with omapdrm in the
following patches, while still leaving the sysfs files out.

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