drm/i915/gt: nuke gen6_hw_id

This is only used by GRAPHICS_VER == 6 and GRAPHICS_VER == 7. All other
recent platforms do not depend on this field, so it doesn't make much
sense to keep it generic like that. Instead, just do a mapping from
engine class to HW ID in the single place that is needed.

v2: use macros with the direct register address instead of calculating
from the legacy HW_ID (Matt Roper)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723002551.3906535-1-lucas.demarchi@intel.com