drm/nouveau: prevent all channel creation if accel not available

Previously, if there was no firmware available, the DRM would just
disable channel creation from userspace, but still use a single
channel for its own purposes.

With a bit of care it should actually be possible to do this, due
to the DRM's very limited use of the engine.  It currently doesn't
work correctly however, resulting in corrupted fbcon and hangs on
a number of cards.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>