drm/xe: Initialize ret in mcr_lock()

ret is not initialized in mcr_lock() when running in platforms with
graphics IP version < 1270, this could cause drm_WARN_ON_ONCE()
to hit eventually(what just happened to me).

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_gt_mcr.c

@@ -383,7 +383,7 @@ static bool xe_gt_mcr_get_nonterminated_steering(struct xe_gt *gt,
 static void mcr_lock(struct xe_gt *gt)
 {
 	struct xe_device *xe = gt_to_xe(gt);
-	int ret;
+	int ret = 0;
 
 	spin_lock(&gt->mcr_lock);