DatePicker: improve form change validation (#12348)

watch.value will keep firing during time picker selection
it now detects whether panel is closed before firing el.form.change

packages/date-picker/src/picker.vue

@@ -429,7 +429,7 @@ export default {
       }
     },
     value(val, oldVal) {
-      if (!valueEquals(val, oldVal)) {
+      if (!valueEquals(val, oldVal) && !this.pickerVisible) {
         this.dispatch('ElFormItem', 'el.form.change', val);
       }
     }
@@ -893,6 +893,7 @@ export default {
       // determine user real change only
       if (!valueEquals(val, this.valueOnOpen)) {
         this.$emit('change', val);
+        this.dispatch('ElFormItem', 'el.form.change', val);
         this.valueOnOpen = val;
       }
     },