drm/xe/pf: Move GGTT config KLVs encoding to helper

In upcoming patch we will want to encode GGTT config KLVs based
on raw numbers, without relying on the allocated GGTT node.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20250711193316.1920-4-michal.wajdeczko@intel.com