drm/i915/xehpsdv: Add maximum sseu limits

Due to the removal of legacy slices and the transition to a
gslice/cslice/mslice/etc. design, we'll internally store all DSS under
"slice0."

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Caz Yokoyama <caz.yokoyama@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210729170008.2836648-10-matthew.d.roper@intel.com