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>

drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c

@@ -289,6 +289,10 @@ int amdgpu_gem_create_ioctl(struct drm_device *dev, void *data,
 	uint32_t handle, initial_domain;
 	int r;
 
+	/* reject DOORBELLs until userspace code to use it is available */
+	if (args->in.domains & AMDGPU_GEM_DOMAIN_DOORBELL)
+		return -EINVAL;
+
 	/* reject invalid gem flags */
 	if (flags & ~(AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED |
 		      AMDGPU_GEM_CREATE_NO_CPU_ACCESS |