drm/nvc0/bar: support chips without BAR3

Adapt the NVC0 BAR driver to make it able to support chips that do not
expose a BAR3. When this happens, BAR1 is then used for USERD mapping
and the BAR alloc() functions is disabled, making GPU objects unable
to rely on BAR for data access and falling back to PRAMIN.

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