drm/i915: Correct ss -> steering calculation for pre-Xe_HP platforms

Accidental use of a "SLICE" macro where a "SUBSLICE" macro was intended
causes the group ID for steering to be calculated incorrectly on
pre-Xe_HP platforms.

Fixes: 9a92732f ("drm/i915/gt: Add general DSS steering iterator to intel_gt_mcr")
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220712220513.3451794-1-matthew.d.roper@intel.com