drm/xe: Add SVM device memory mirroring

Add SVM device memory mirroring which enables device pages for
migration. Enabled via CONFIG_XE_DEVMEM_MIRROR Kconfig. Kconfig option
defaults to enabled. If not enabled, SVM will work sans migration and
KMD memory footprint will be less.

v3:
 - Add CONFIG_XE_DEVMEM_MIRROR
v4:
 - Fix Kconfig (Himal)
 - Use %pe to print errors (Thomas)
 - Fix alignment issue (Checkpatch)
v5:
 - s/xe_mem_region/xe_vram_region (Rebase)
v6:
 - Only compile if CONFIG_DRM_GPUSVM selected (CI, Lucas)
 - s/drm_info/drm_dbg/

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>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306012657.3505757-22-matthew.brost@intel.com