drm/nouveau/graph: enable when using external fw

nvc0_graph_ctor() would only let the graphics engine be enabled if its
oclass has a proper microcode linked to it. This prevents GR from being
enabled at all on chips that rely exclusively on external firmware, even
though such a use-case is valid.

Relax the conditions enabling the GR engine to also include the case
where an external firmware has also been loaded.

Also switch to external firmware if the graph class has no microcode
linked to it.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>