drm/i915/gt: rename legacy engine->hw_id to engine->gen6_hw_id

We kept adding new engines and for that increasing hw_id unnecessarily:
it's not used since GRAPHICS_VER == 8. Prepend "gen6" to the field and
try to pack it in the structs to give a hint this field is actually not
used in recent platforms.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210720232014.3302645-4-lucas.demarchi@intel.com