drm/xe: Validate preferred system memory placement in xe_svm_range_validate

drm/xe: Validate preferred system memory placement in xe_svm_range_validate

Ensure preferred system memory placement is checked in
xe_svm_range_validate when dpagemap is NULL. Without this check, a
prefetch to system memory may become a no-op because device memory is
considered a valid placement.

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: 238dbc9d9f4a ("drm/xe: Use the vma attibute drm_pagemap to select where to migrate")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patch.msgid.link/20260106213443.1866797-1-matthew.brost@intel.com