Placed render targets are not initialized properly in D3D12

Run with -dx12 -d3ddebug on the command line. Observe this validation error:

LogD3D12RHI: Error: [D3DDebug] ID3D12CommandQueue1::ExecuteCommandLists: Placed resources, reserved resources, or committed resources with D3D12_HEAP_FLAG_CREATE_NOT_ZEROED flag with either render target or depth stencil flags must be initialized with a Discard/Clear/Copy operations before other operations are supported. Resource (0x000002257BFBD880:'CombineLUTs'), Subresource (0) is not initialized but is used in Function (ID3D12CommandList::DrawIndexedInstanced) on Command List (0x00000224A6C8E870:'FD3D12CommandList (GPU 0)').