drm/xe/vm: use list_last_entry() to fetch last_op

drm/xe/vm: use list_last_entry() to fetch last_op

I would imagine that it's more efficient to fetch ops_list->prev than
to walk the whole list forward.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>