drm/i915/dg1: Add DG1 PCI IDs

Add the PCI ID for DG1, but keep it out of the table we use to register
the driver. At this point we can't consider the driver ready to bind to
the device since we basically miss support for everything. When more
support is merged we can enable it to work partially for example as a
display-only driver.

v2: remove DG1 from the pci table and reword commit message (Lucas)

Bspec: 44463

Cc: Matthew Auld <matthew.auld@intel.com>
Cc: James Ausmus <james.ausmus@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com> # v1
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200713182321.12390-3-lucas.demarchi@intel.com