drm/i915/tgl: add gen12 to stolen initialization

Add case for gen == 12 and add MISSING_CASE() for future gens. We were
already handling gen12 as the default, so this doesn't change the
current behavior.

BSpec: 19481 and 44980

Cc: CQ Tang <cq.tang@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190904213419.27547-7-jose.souza@intel.com
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>