fix(datepicker): use default cursor on toggle when disabled (#4604)

Author: willshowell

src/lib/datepicker/datepicker-toggle.scss

@@ -11,5 +11,8 @@ $mat-datepicker-toggle-icon-size: 24px !default;
   border: none;
   outline: none;
   vertical-align: middle;
-  cursor: pointer;
+
+  &:not([disabled]) {
+    cursor: pointer;
+  }
 }