viewportTopR should always have 'auto' scroll

This fixes an issue when you have the following configuration

`
options: {
  frozenColumn: 0,
  frozenRow: 1,
  frozenBottom: true
}
`

Basically, when you have a frozen bottom row w/ a frozen column, AND you have less rows than would fill up the height of the viewport, a disabled scrollbar is shown for the top right viewport, and it also visually overlaps the rightmost column.