DRM/xe: Don't hand out the flat CCS storage as usable VRAM

  [ And this was a debug session from hell, enormously helped by an AI
    doing much of the grunt-work.
  
    I'd like to call it my tireless helper, but the AI several times
    stated flat out that this was impossible and unsolvable and that we
    should just write a report about it.
  
    I suspect those things have been trained by people who may not be
    quite as stubborn as I am.
  
    But while the AI was ready to give up several times, it did keep
    adding debug code and analyzing it faithfully when I pushed. So credit
    where credit is due and I let the AI write the commit message above.
  
    This is basically a one-liner fixing a bogus "round_up()" to a
    "round_down()", but there were 24 patches adding more and more debug
    information to this, and 18 kernel boot to finally narrow it down to
    this.   - Linus ]