drm/xe: Make the PT code handle placement per PTE rather than per vma / range

drm/xe: Make the PT code handle placement per PTE rather than per vma / range

With SVM, ranges forwarded to the PT code for binding can, mostly
due to races when migrating, point to both VRAM and system / foreign
device memory. Make the PT code able to handle that by checking,
for each PTE set up, whether it points to local VRAM or to system
memory.

v2:
- Fix system memory GPU atomic access.
v3:
- Avoid the UAPI change. It needs more thought.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://lore.kernel.org/r/20250326080551.40201-6-thomas.hellstrom@linux.intel.com