Add set_output/unset_output ops for overlay managers, these form links between
managers and outputs. Create a function in dss features which tell all the
output instances that connect to a manager, use it when a manager tries to set
an output. Add a constraint of not unsetting an output when the manager is
enabled.

Keep the omap_dss_device pointer and set/unset_device ops in overlay_manager for
now to not break things. Keep the dss feature function get_supported_displays
as it's used in some places. These will be removed later.
Signed-off-by: Archit Taneja <archit@ti.com>