drm/i915/dg1: Add fake PCH

DG1 has the south engine display on the same PCI device. Ideally we
could use HAS_PCH_SPLIT(), but that macro is misused all across the
code base to rather signify a range of gens. So add a fake one for DG1
to be used where needed.

Cc: Aditya Swarup <aditya.swarup@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200713182321.12390-6-lucas.demarchi@intel.com