fix(DropdownItem): set transform-origin (#3953)

Author: anlyyao

packages/components/popup/popup.less

@@ -73,6 +73,7 @@
   &.@{prefix}-fade-leave-to {
     &.@{popup}--top {
       transform: translateY(-100%);
+      transform-origin: 0% 0%;
     }
 
     &.@{popup}--bottom {