drm/xe: Make xe_svm_alloc_vram public

drm/xe: Make xe_svm_alloc_vram public

This function will be used in prefetch too, hence make it public.

v2:
  - Add kernel-doc (Matthew Brost)
  - Rebase

v3:
 - Move CONFIG_DRM_XE_DEVMEM_MIRROR stub out to xe_svm.c (Matthew Brost)

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://lore.kernel.org/r/20250513040228.470682-3-himal.prasad.ghimiray@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>