r600g: add back u_upload_mgr integration

r600g: add back u_upload_mgr integration

I can't see a performance difference with this code, which means all
the driver-specific code removed in this commit was unnecessary.

Now we use u_upload_mgr in a slightly different way than we did before it got
dropped. I am not restoring the original code "as is" due to latest
u_upload_mgr changes that r300g performance benefits from.

This also fixes:
- piglit/fp-kil