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