drm: xlnx: zynqmp_dpsub: Move encoder to DPSUB core

As part of the transitition of the DP encoder to a DRM bridge, turn the
DRM encoder into a dummy encoder and move it out of the DP code, to the
DPSUB core. DP encoder operations are handled by the DP bridge, which is
now attached to the encoder.

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