cuda4dnn: fuse activations with convolutions

* fuse ReLU, ReLU6, TanH, Sigmoid with conv

* fix OpenCL errors

* improve ReLU, add power, swish and mish

* fix missing fusion entries

* fix handling of unsetAttached

* remove whole file indentation

* optimize power = 1.0, use IDENTITY instead of NONE

* handle edge case: change backend and then clear