make skip_if_no_gpu more robust

Seems cuda.cuda_available is only checking for cuda availability. 

```
$ THEANO_FLAGS=device=gpu python -c 'from theano.sandbox import cuda; print cuda.cuda_available'
Using gpu device 2: GeForce GTX 285
True

$ THEANO_FLAGS=device=cpu python -c 'from theano.sandbox import cuda; print cuda.cuda_available'
True
```
But we need skip_if_no_gpu to skip if even if cuda is installed but gpu is not enabled or available. This PR fixes this issue.