SystemUI: Avoid adding useless empty quick settings page

* Previously the code would work this way:
  if nTiles == nPages * nRows * nCols we aren't
  adding additional page but otherwise we add a
  single empty page.
* Now that I'm basically always rounding
  nTiles / (nPages * nRows * nCols) the numPages
  variable is initialized to proper value.

Change-Id: Ie352f51d985f3ed97dc645bc18645894201998ff