drm/i915/xehp: Loop over all gslices for INSTDONE processing

We no longer have traditional slices on Xe_HP platforms, but the
INSTDONE registers are replicated according to gslice representation
which is similar.  We can mostly re-use the existing instdone code with
just a few modifications:

 * Create an alternate instdone loop macro that will iterate over the
   flat DSS space, but still provide the gslice/dss steering values for
   compatibility with the legacy code.

 * We should allocate INSTDONE storage space according to the maximum
   number of gslices rather than the maximum number of legacy slices to
   ensure we have enough storage space to hold all of the values.  XeHP
   design has 8 gslices, whereas older platforms never had more than 3
   slices.

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/20210805163647.801064-3-matthew.d.roper@intel.com