drm: xlnx: zynqmp_dpsub: Register AUX bus at bridge attach time

To prepare for operating as a standalone DP bridge with the DRM device
implemented in the PL, move registration of the AUX bus to bridge attach
time, as that's the earliest point when a DRM device is available.

The DRM device pointer stored in zynqmp_dp isn't used anymore, drop it.

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