drm/gpuva: Add drm_gpuva_for_each_op_reverse

Add a helper to walk op list in reverse. Xe will make use of this when
unwinding GPUVA operations.

v2: (Rodrigo) reword commit message

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>