drm/xe: Add Xe SVM populate_devmem_pfn GPU SVM vfunc

Get device pfns from BO's buddy blocks. Used in migrate_* core MM
functions called in GPU SVM to migrate between device and system memory.

v2:
 - Use new drm_gpusvm_devmem_ops
v3:
 - Better commit message (Thomas)
v5:
 - s/xe_mem_region/xe_vram_region (Rebase)

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Oak Zeng <oak.zeng@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306012657.3505757-26-matthew.brost@intel.com