fix(text-input): correct disabled placeholder styles, all browsers

Author: Cathy Siller

src/helix-ui/styles/components/text-inputs.less

@@ -34,6 +34,7 @@
   }
 
   &[disabled] {
+    #Mix.disabledPlaceholder();
     background-color: @gray-50;
     border-color: @gray-300;
     border-width: 1px;