drm/amd/amdgpu: validate the shadow BO.

Fixes a rare NULL pointer dereference in amdgpu_ttm_bind.

The issue was found by Nicolai Haehnle.
The patch was tested by Nicolai Haehnle.

Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org