drm/i915/gt: Document function to decode register state context

It's not obvious how the encode/decode of the per platform tables is
done. Document it so while adding tables for new platforms people can be
confident they right things is being done.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220930050903.3479619-3-lucas.demarchi@intel.com