OMAPDSS: add manager ops

The output drivers need some operations from the overlay managers, like
enable and set_timings. These will affect the dispc registers, and need
to be synchronized with the composition-side changes with overlays and
overlay managers.

We want to handle these calls in the apply.c in the compatibility mode,
but when in non-compat mode, the calls need to be handled by some other
component (e.g. omapdrm).

To make this possible, this patch creates a set of function pointers in
a dss_mgr_ops struct, that is used to redirect the calls into the
correct destination.

The non-compat users can install their mgr ops with
dss_install_mgr_ops() function.

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