dkms updates

In preparation for changing the dkms package layout (e.g. splitting off a dh-dkms package), please switch to dh-sequence-dkms. (dh-sequence-dkms is already available in bullseye and buster-backports.)

Also let's enable the default autopkgtests for *-dkms packages. With the latest dkms upload the autopkgtest will finally try to install the missing kernel headers and compile the kernel modules, so we can quickly notice if out-of-tree modules get broken by a kernel update.

I verified that the provided commits do not cause any other changes to the binary packages and that the autopkgtests passed in sid/amd64.