Some new Loongson servers are using Aspeed BMC, which has an GPU.
Some other Loongson servers are using SM750 GPU instead of AMD's.
Since MIPS doesn't have a generic display driver like VESA, we need
to install sm750fb and (drm_)ast into Loongson's fb-moduels udeb package.
(cherry picked from commit 6fbe9f4e36)