Avoid rebuilds of dkms package

Hi, currently the nvidia-open and nvidia-open-dkms packages are always built together - every time the default kernel (linux package) updates. However that means the the dkms package does get unnecessarily rebuilt all the time and users regularly get updates for it even if nothing changed and they are not even using the default kernel.

It would be cool to split the packages and build nvidia-open-dkms only if actually something changed (and not force the update cycle of the default kernel on users of other kernels).