drm/xe/svm: Fix a potential bo UAF

If drm_gpusvm_migrate_to_devmem() succeeds, if a cpu access happens to the
range the bo may be freed before xe_bo_unlock(), causing a UAF.

Since the reference is transferred, use xe_svm_devmem_release() to
release the reference on drm_gpusvm_migrate_to_devmem() failure,
and hold a local reference to protect the UAF.

Fixes: 2f118c94 ("drm/xe: Add SVM VRAM migration")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250326080551.40201-3-thomas.hellstrom@linux.intel.com