drm/xe: Assert G2H outstanding when releasing G2H

Ensure we are managing G2H credits correctly. Extra important now that
this is tied to PM.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240725231801.1958038-1-matthew.brost@intel.com