drm/i915/dg1: add initial DG-1 definitions

Bspec: 33617, 33617

v2: s/intel_dg1_info/dg1_info/ as done for other platforms before and
    try to shut up compiler about ununsed variable that we know
    shouldn't be used (Lucas)
v3: replace explicit attribute with __maybe_unused (Lucas)

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Stuart Summers <stuart.summers@intel.com>
Cc: Vanshidhar Konda <vanshidhar.r.konda@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Aravind Iddamsetty <aravind.iddamsetty@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>
Link: https://patchwork.freedesktop.org/patch/msgid/20200713182321.12390-2-lucas.demarchi@intel.com