drm/i915/xehp: Correct steering initialization

Another mistake during the conversion to DSS bitmaps:  after retrieving
the DSS ID intel_sseu_find_first_xehp_dss() we forgot to modulo it down
to obtain which ID within the current gslice it is.

Fixes: b87d3901 ("drm/i915/sseu: Disassociate internal subslice mask representation from uapi")
Cc: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220607175716.3338661-1-matthew.d.roper@intel.com

drivers/gpu/drm/i915/gt/intel_workarounds.c

@@ -1177,8 +1177,8 @@ xehp_init_mcr(struct intel_gt *gt, struct i915_wa_list *wal)
 	}
 
 	slice = __ffs(slice_mask);
-	subslice = intel_sseu_find_first_xehp_dss(sseu, GEN_DSS_PER_GSLICE, slice);
-	WARN_ON(subslice > GEN_DSS_PER_GSLICE);
+	subslice = intel_sseu_find_first_xehp_dss(sseu, GEN_DSS_PER_GSLICE, slice) %
+		GEN_DSS_PER_GSLICE;
 
 	__add_mcr_wa(gt, wal, slice, subslice);