drm/nouveau/device: return error for unknown chipsets

Previously the code relied on device->pri to be NULL and to fail probing
later. We really should just return an error inside nvkm_device_ctor for
unsupported GPUs.

Fixes: 24d5ff40 ("drm/nouveau/device: rework mmio mapping code to get rid of second map")

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Cc: dann frazier <dann.frazier@canonical.com>
Cc: dri-devel <dri-devel@lists.freedesktop.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: stable@vger.kernel.org
Reviewed-by: Jeremy Cline <jcline@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006220528.13925-1-kherbst@redhat.com