drm: xlnx: zynqmp_dpsub: Make drm bridge discoverable

ZynqMP DPSUB supports 2 input modes: DMA based and live video.

In the first mode, the driver implements CRTC and DP encoder DRM bridge
to model the complete display pipeline. In this case, DRM bridge is
being directly instantiated within the driver, not using any bridge
discovery mechanisms.

In the live video input mode video signal is generated by FPGA fabric
and passed into DPSUB over the connected bus. In this mode driver
exposes the DP encoder as a DRM bridge, expecting external CRTC to
discover it via drm_of_find_panel_or_bridge() or a similar call. This
discovery relies on drm_bridge.of_node being properly set.

Assign device OF node to the bridge prior to registering it. This will
make said bridge discoverable by an external CRTC driver.

Signed-off-by: Anatoliy Klymenko <anatoliy.klymenko@amd.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240124025402.373620-2-anatoliy.klymenko@amd.com