drm/xe/guc: Only take actions in CT irq handler if CTs are enabled

Protect entire IRQ handler by CT being enabled rather than just G2H
handler.

v2: Return on not enabled in CT irq handler (Michal)

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>