drm/xe: Do not warn on SVM migration failing because of 64k requirements

On platforms which only support 64k VRAM pages, it is expected that 4k
faults will not migrate. Do not warn on this, rather print a debug
message.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250529164338.1745515-1-matthew.brost@intel.com