util/u_threaded: fix a CPU storage crash due to an invalid codepath taken
Fixes:
3df9d8ed807 - gallium/u_threaded: implement pipelined partial buffer uploads using CPU storage
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6837
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6594
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6613
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17556>