drm/xe: Select DRM_GPUSVM Kconfig

Xe depends on DRM_GPUSVM for SVM implementation, select it in Kconfig.

v6:
 - Don't select DRM_GPUSVM if UML (CI)
v7:
 - Only select DRM_GPUSVM if DEVICE_PRIVATE (CI)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306012657.3505757-8-matthew.brost@intel.com