drm/i915: add new gen12 dgfx platform macro

Add a new macro for GEN12 platforms to be grouped under dgfx feature
set.

Signed-off-by: Stuart Summers <stuart.summers@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/20191024195122.22877-2-lucas.demarchi@intel.com