drm: xlnx: zynqmp_dpsub: Attach to the next bridge

The next component in the display chain, after the DP encoder, is most
likely a DP connector. The display connector driver registers a bridge
for it. That bridge doesn't need to be controlled, but is needed in
order to use the DRM connector bridge helper. Retrieve it at init time,
and attach to it in the DP bridge attach handler.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>