This pipe cap is for partial primitive restart support. According to the documentation:
PIPE_CAP_PRIMITIVE_RESTART_FIXED_INDEX: Subset of PRIMITIVE_RESTART where the restart index is always the fixed maximum value for the index type.
If your driver exports it, you should not currently be exporting
PIPE_CAP_PRIMITIVE_RESTART, as they conflict. And even if you do export it, it does nothing because you're already exporting the superset cap.
The only driver in the entire tree which got this right is v3d. Good job v3d.
Nope, v3d gets a special cone of shame for being the only driver to only export PIPE_CAP_PRIMITIVE_RESTART_FIXED_INDEX and then also handling non-fixed restart index.