OMAPDRM: fix overlay manager handling

Currently omapdrm creates crtcs, which map directly to DSS overlay
managers, only on demand at init time. This would make it difficult to
manage connecting the display entities in the future, as the code cannot
just search for a suitable overlay manager.

We cannot fix this the sane way, which would be to create crtcs for each
overlay manager, because we need an overlay for each crtc. With limited
number of overlays, that's not possible.

So the solution for now is to detach the overlay manager from the crtc.
crtcs are still created on demand at init time, but all overlay managers
are always initialized by the omapdss.

This way we can create and connect whole display pipelines from the
overlay manager to the display, regardless of which crtcs omapdrm would
create.

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