drm/amdgpu: don't allow userspace to create a doorbell BO

We need the domains in amdgpu_drm.h for the kernel driver to manage
the pool, but we don't want userspace using it until the code
is ready.  So reject for now.

Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>