gallium/u_index_modify: don't add PIPE_TRANSFER_UNSYNCHRONIZED unconditionally

It's OK for r300g (because r300g can't write to buffers via the GPU), but
not later hardware. This issue was spotted randomly.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
(cherry picked from commit c8ef5123980f9f538c79e626b0092660a2256ae6)
5 files changed