drm/amdgpu: fix VM clearing in amdgpu_gem_object_close

We need to check if the VM is swapped out before trying to update it.

Fixes: 23e0563e ("drm/amdgpu: clear freed mappings immediately when BO may be freed")
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>