D3D11: Validation error during buffer upload

There is a D3D11 validation error when uploading buffers:

```
D3D11 ERROR: ID3D11DeviceContext::CopySubresourceRegion: Cannot invoke CopySubresourceRegion while a Subresource of the source is mapped.Source subresource (0) is mapped. [ RESOURCE_MANIPULATION ERROR #283: COPYSUBRESOURCEREGION_INVALIDSOURCESTATE]
```

It's easy to reproduce by running any of the `cmd-upload-buffer-*` tests.