drm/xe/guc: Update few GuC CTB ABI definitions

In upcoming new GuC ABI definitions we will want to refer to max
number of dwords that could fit into CTB HXG message. Add explicit
definition named as GUC_CTB_MAX_DWORDS and start using it.

Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20240104222031.277-6-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>