drm/i915/dg2: Add fake PCH

As with DG1, DG2 has an ICL-style south display interface provided on
the same PCI device.  Add a fake PCH to ensure DG2 takes the appropriate
codepaths for south display handling.

Bspec: 54871, 50062, 49961, 53673
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Aditya Swarup <aditya.swarup@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721223043.834562-11-matthew.d.roper@intel.com