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 switch to the generic 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. No more need for home-grown solutions ;-)

As the modules cannot be built for PREEMPT_RT kernels, simply blacklist them in dkms.conf.