drm: xlnx: zynqmp_dpsub: Decouple DRM device from zynqmp_dpsub

To complete the decoupling of the DRM device from the zynqmp_dpsub,
group all DRM-related structures in a zynqmp_dpsub_drm structure and
allocate it separately from the zynqmp_dpsub. The DRM managed allocation
of the drm_device now doesn't cover the zynqmp_dpsub anymore, so we need
to register a cleanup action to release the zynqmp_dpsub when the
drm_device is released.

The will allow usage of the DisplayPort encoder as a standalone bridge,
without registering a DRM device in this driver.

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