As an alternative to building a kernel, there is always the possibility of making an out-of-tree build of the nouveau kernel module, see § 2.2 here
. Just check that the running kernel be recent enough as told.
In addition, you could wish to upgrade the nouveau X kernel module. To do that you would just have to grab the most recent tarball here
and use xf86-video-nouveau.Slackbuild in /testing to build a new package. This usually works, but in some cases you would have to upgrade some other components as well, libdrm and mesa being the usual culprits. But, I digress...
As for updating -current every time a device is better supported by some kernel module, that is not very realistic IMHO as that would mean updating -current every time a new mainline kernel is released.