[powerpc/powerpc64,ppc64el] udeb: add DRM ast driver to fb-modules (Closes: #990016)
OpenPOWER systems do not have a fallback VGA BIOS, and thus require a Linux framebuffer driver for the installer to display. This was already enabled in arm64.
Fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=990016