drm/xe: Fix uninitialized return value from xe_validation_guard()

drm/xe: Fix uninitialized return value from xe_validation_guard()

the DEFINE_CLASS() macro creates an inline function and
the init args are passed down to it; since _ret is passed as an int,
whatever value is set inside the function is not visible to the caller.
Pass _ret as a pointer so its value propagates to the caller.

Fixes: c460bc2311df ("drm/xe: Introduce an xe_validation wrapper around drm_exec")
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6220
Cc: Maarten Lankhorst <maarten.lankhorst@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: intel-xe@lists.freedesktop.org
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251027131228.12098-1-thomas.hellstrom@linux.intel.com