ubuntu-drivers autoinstall --- UnboundLocalError

[Lorenzo Moriondo]

* I have gone through `do-release-upgrade` and the system has upgraded successfully.
* I have rolled back the graphic drivers to nouveau to have a clean drivers installation

Once I tried to run `$ sudo ubuntu-drivers autoinstall` to install graphics drivers, this exception was raised:
```
Traceback (most recent call last):
  File "/usr/bin/ubuntu-drivers", line 513, in <module>
    greet()
  File "/usr/lib/python3/dist-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python3/dist-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python3/dist-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python3/dist-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/click/decorators.py", line 84, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/usr/bin/ubuntu-drivers", line 432, in autoinstall
    command_install(config)
  File "/usr/bin/ubuntu-drivers", line 187, in command_install
    UbuntuDrivers.detect.nvidia_desktop_pre_installation_hook(to_install)
  File "/usr/lib/python3/dist-packages/UbuntuDrivers/detect.py", line 839, in nvidia_desktop_pre_installation_hook
    with_nvidia_kms = version >= 470
UnboundLocalError: local variable 'version' referenced before assignment
```

It looks like `version` is an unitialized variable, Cheers.