drm/nouveau: fixup fbcon failure paths

Add missing calls, and fix a leak from forgetting to call the unpin function.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>