OMAPDSS: output: Add set/unset device ops for omap_dss_output

An output entity represented by the struct omap_dss_output connects to a
omap_dss_device entity. Add functions to set or unset an output's device. This
is similar to how managers and devices were connected previously. An output can
connect to a device without being connected to a manager. However, the output
needs to eventually connect to a manager so that the connected panel can be
enabled.

Keep the omap_overlay_manager pointer in omap_dss_device for now to prevent
breaking things. This will be removed later when outputs are supported
completely.

Signed-off-by: Archit Taneja <archit@ti.com>