OMAPDSS: DSS: Add MEM2MEM manager mode

This change adds memory-to-memory (M2M) mode with using of the
manager. The main idea is to connect write back (WB) pipeline to
the manager (overlay, in terms of technical reference manual)
which is not in use (physical interface is turned-off) for using
advantages of hardware manager in M2M mode. In this case several
pipelines can be connected to the manager.

Change-Id: Ifa649bd5b18f54f3ce583fdee802c799b2c30711
Co-author: Sergiy Kibrik <sergiikibrik@ti.com>
Signed-off-by: Mykhailo Denysiuk <x0172934@ti.com>