drm/i915: Introduce G12 subplatform of DG2

Another fork of the DG2 design has appeared, known as "DG2-G12;" let's
add it as a new subplatform.  As with G11, the GT stepping resets back
to A0 (so a DG2-G12 A0 is similar, but not identical, to a DG2-G10 C0)
but the display steppings continue to use the same numbering scheme as
G10 and G11.

Some existing DG2 workarounds are starting to be extended to the DG2-G12
subplatform.  So far only workarounds that were "permanent" for both
DG2-G10 and DG2-G11 have been tagged for DG2-G12, but more
stepping-specific workarounds are likely to show up in the future.

Bspec: 44477
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220120235016.1209326-1-matthew.d.roper@intel.com