u_upload_mgr: new features

u_upload_mgr: new features

- Added a parameter to specify a minimum offset that should be returned.
  r300g needs this to better implement user buffer uploads. This weird
  requirement comes from the fact that the Radeon DRM doesn't support negative
  offsets.

- Added a parameter to notify a driver that the upload flush occured.
  A driver may skip buffer validation if there was no flush, resulting
  in a better performance.

- Added a new upload function that returns a pointer to the upload buffer
  directly, so that the buffer can be filled e.g. by the translate module.