drm/xe: Add a NULL check in xe_ttm_stolen_mgr_init

Add an explicit check to ensure that the mgr is not NULL.

Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240319130925.22399-1-nirmoy.das@intel.com

drivers/gpu/drm/xe/xe_ttm_stolen_mgr.c

@@ -207,6 +207,11 @@ void xe_ttm_stolen_mgr_init(struct xe_device *xe)
 	u64 stolen_size, io_size, pgsize;
 	int err;
 
+	if (!mgr) {
+		drm_dbg_kms(&xe->drm, "Stolen mgr init failed\n");
+		return;
+	}
+
 	if (IS_SRIOV_VF(xe))
 		stolen_size = 0;
 	else if (IS_DGFX(xe))