drm/xe/mcr: Try to derive dss_per_grp from hwconfig attributes

When steering MCR register ranges of type "DSS," the group_id and
instance_id values are calculated by dividing the DSS pool according to
the size of a gslice or cslice, depending on the platform.  These values
haven't changed much on past platforms, so we've been able to hardcode
the proper divisor so far.  However the layout may not be so fixed on
future platforms so the proper, future-proof way to determine this is by
using some of the attributes from the GuC's hwconfig table.  The
hwconfig has two attributes reflecting the architectural maximum slice
and subslice counts (i.e., before any fusing is considered) that can be
used for the purposes of calculating MCR steering targets.

If the hwconfig is lacking the necessary values (which should only be
possible on older platforms before these attributes were added), we can
still fall back to the old hardcoded values.  Going forward the hwconfig
is expected to always provide the information we need on newer
platforms, and any failure to do so will be considered a bug in the
firmware that will prevent us from switching to the buggy firmware
release.

It's worth noting that over time GuC's hwconfig has provided a couple
different keys with similar-sounding descriptions.  For our purposes
here, we only trust the newer key "70" which has supplanted the
similarly-named key "2" that existed on older platforms.

Bspec: 73210
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240815172602.2729146-4-matthew.d.roper@intel.com

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