drm/xe/guc: Clear whole g2h_fence during initialization

The struct g2h_fence must be explicitly initializated using the
g2h_fence_init() function to avoid trash values in its members,
but we missed to update this helper function with the new member.

To fix that and avoid any future mistakes, memset the whole struct
first, then update remaining non-zero members.

Fixes: 94de94d2 ("drm/xe/guc: Cancel ongoing H2G requests when stopping CT")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Lukasz Laguna <lukasz.laguna@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250723175639.206875-1-michal.wajdeczko@intel.com