drm/xe/guc: Start handling GuC Relay event messages

GuC Relay infrastructure is ready, start handling relay messages
from the GuC to unblock testing on the live system.

Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20240104222031.277-11-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>