Set lowDPI tiles to be the default

Since HighDPI tiles are currently causing severe performance penalties
on mobile devices (at least iOS devices), this patch sets the default
mode to highdpi = false.

Change-Id: I2f99e3955ecccd905e8962d005421d1b5276ea84
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>