drm/i915/gt: nuke unused legacy engine hw_id

The engine hw_id is only used by RING_FAULT_REG(), which is not used
by GRAPHICS_VER >= 8. We did use hw_id on recent platforms to set
the engine's guc_id, but that is not the case anymore since
commit c784e524 ("drm/i915/guc: Update to use firmware v49.0.1"):
now we only use class and id information to generate guc_id.

We tend to keep adding new defines just to be consistent, but let's try
to remove them and let them defined to 0 for engines that only exist on
gen8+ platforms.

v2: Reword commit message and add information about when we stopped
using hw_id (Matt Roper)

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/20210720232014.3302645-3-lucas.demarchi@intel.com