drm/xe/guc: Add helpers for HXG messages

In addition to MMIO and CTB communication between the host driver
and the GUC firmware, we will start using GuC HXG message protocol
in communication between SR-IOV VFs and PF. Define helpers related
to HXG message protocol to minimize code duplication.

Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20240104222031.277-5-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>