drm: xlnx: zynqmp_dpsub: Move connector registration to bridge attach

Connector creation requires the DRM encoder, and it thus typically
performed in the bridge attach operation. Move it there, to prepare for
registration of the DRM bridge. For now the zynqmp_dp_bridge_attach() is
called manually at initialization time.

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