drm/xe: Handle unmapped userptr in analyze VM

A corner exists where a userptr may have no mapping when analyze VM is
called, handle this case.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>