drm/xe: Use the vma attibute drm_pagemap to select where to migrate

drm/xe: Use the vma attibute drm_pagemap to select where to migrate

Honor the drm_pagemap vma attribute when migrating SVM pages.
Ensure that when the desired placement is validated as device
memory, that we also check that the requested drm_pagemap is
consistent with the current.

v2:
- Initialize a struct drm_pagemap pointer to NULL that could
  otherwise be dereferenced uninitialized. (CI)
- Remove a redundant assignment (Matt Brost)
- Slightly improved commit message (Matt Brost)
- Extended drm_pagemap validation.

v3:
- Fix a compilation error if CONFIG_DRM_GPUSVM is not enabled.
  (kernel test robot <lkp@intel.com>)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patch.msgid.link/20251219113320.183860-14-thomas.hellstrom@linux.intel.com