Check left/top flag instead of uiNeighborAvail in neon deblocking

When deblocking over slice edges (in single threaded sliced
encoding), uiNeighborAvail might say that a MB isn't available,
even though it is available for deblocking here.

This fixes the encoder test of dynamic slicing, with neon
optimizations enabled.

Review at https://rbcommons.com/s/OpenH264/r/628/.