drm/xe: Add helpers to hide struct xe_vma internals

This will help with the GPUVA port as the internals of struct xe_vma
will change.

v2: Update comment around helpers

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.kernel.org>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>