drm/xe/xe_sync: initialise ufence.signalled

We can incorrectly think that the fence has signalled, if we get a
non-zero value here from the kmalloc, which is quite plausible. Just use
kzalloc to prevent stuff like this.

Fixes: 977e5b82 ("drm/xe: Expose user fence from xe_sync_entry")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: <stable@vger.kernel.org> # v6.10+
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241011133633.388008-2-matthew.auld@intel.com