riscv: defconfig: enable DRM_NOUVEAU

Both RADEON and NOUVEAU graphics cards are supported on RISC-V. Enabling
the one and not the other does not make sense.

As typically at most one of RADEON, NOUVEAU, or VIRTIO GPU support will be
needed DRM drivers should be compiled as modules.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>