drm: xlnx: zynqmp_dpsub: Create DRM bridge to model DP encoder

The DP encoder is currently modelled as a DRM encoder and DRM connector.
This doesn't support system configurations where the DP encoder is
driven by the FPGA programmable logic, using the live video input to the
DP subsystem. To enable such use cases, we need to model the encoder as
a DRM bridge.

As a first step, create a DRM bridge in the DP encoder driver. Move and
delegate the implementation of the DRM encoder and connector operations
to the bridge to prepare for the transition. The bridge will be
registered with the DRM core as a separate change.

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