drm/xe/guc: Prefer GT oriented asserts in CTB code

GuC CTB is related to the GT, so best to use xe_gt_assert().

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404193647.759-2-michal.wajdeczko@intel.com