These features are complicated to support on GPU and hardly compatible
with subdivision in the first place. In the future, with T68891 and
T68893, subdivision and custom smooth shading will be separate workflows.
For now, and to better prepare for this future (although long term
plan), we should discourage workflows mixing subdivision and custom
smooth normals, and as such, this disables GPU subdivision when
autosmoothing or custom split normals are used.

This also adds a message in the modifier's UI to indicate that GPU
subdivision will be disabled if autosmooth or custom split normals are
used on the mesh.