kernel - Increase DMA reserve from 16M to 128M by default

kernel - Increase DMA reserve from 16M to 128M by default

* People running DragonFly on workstations were having to specify
  more than the default 16M for vm.dma_reserved in /boot/loader.conf
  or their X sessions would not be stable.

* To reduce confusion, the dma_reserved default is being increased
  to 128M which should be sufficient for most display setups.

  People with headless servers will have to explicitly reduce the
  reservation in /boot/loader.conf (back to 16m is my suggestions) if
  they wish to recover the memory.

* This is the best compromise I could think of.  We can't just return
  the memory to the pool after boot because X might be started far later
  on, or even potentially killed and restarted.  Other drivers might also
  depend on large swaths of contiguous physical memory being available.

  The reserve is the best way to do it and I would rather things work out
  of the box rather than forcing regular users to set something in
  /boot/loader.conf.