drm/i915: split dgfx features from gen 12

Make them independent so we can use DGFX_FEATURES more generically.
For future platforms that do not use the GEN nomenclature we will define
graphics, media and display separately, so we avoid setting graphics_ver
with the GEN() macro.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210413051002.92589-13-lucas.demarchi@intel.com