Enable the dpsub device and wire it up to the PS-GTR PHY lanes routed to
the DisplayPort connector.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/9769d4d103b6eb75e3324825117f6832a746004e.1611232558.git.michal.simek@xilinx.com