drm/xe: Return error if drm_buddy_init() fails

Fix warning:
	../drivers/gpu/drm/xe/xe_ttm_vram_mgr.c: In function ‘__xe_ttm_vram_mgr_init’:
	../drivers/gpu/drm/xe/xe_ttm_vram_mgr.c:340:13: error: variable ‘err’ set but not used [-Werror=unused-but-set-variable]
	  340 |         int err;
	      |             ^~~

Check for the error return and return it, like done by other drivers.
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_ttm_vram_mgr.c

@@ -348,6 +348,8 @@ int __xe_ttm_vram_mgr_init(struct xe_device *xe, struct xe_ttm_vram_mgr *mgr,
 
 	ttm_resource_manager_init(man, &xe->ttm, size);
 	err = drm_buddy_init(&mgr->mm, man->size, default_page_size);
+	if (err)
+		return err;
 
 	ttm_set_driver_manager(&xe->ttm, mem_type, &mgr->manager);
 	ttm_resource_manager_set_used(&mgr->manager, true);