drm/amdgpu: Correct unlocked update fence handling

To fix two issues with unlocked update fence:

1. vm->last_unlocked store the latest fence without taking refcount.
2. amdgpu_vm_bo_update_mapping returns old fence, not the latest fence.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>