drm/xe/guc: Use HXG definitions on HXG messages

While parsing and processing CTB G2H messages we should extract
underlying HXG message and use HXG definitions on such message.
Using outer CTB layer message in HXG definitions require use of
shifted dword index, which might be confusing:

	FIELD_GET(GUC_HXG_MSG_0_xxx, msg[1])

instead of:

	FIELD_GET(GUC_HXG_MSG_0_xxx, hxg[0])

Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20240111210632.717-1-michal.wajdeczko@intel.com


Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>