gallium/u_upload_mgr: lower risk of hitting an assert
The assert(size < INT32_MAX / 2) can be triggered by large uploads.
Since we know that the caller of u_upload_alloc_buffer will consume min_size bytes, we can init buffer_private_refcount to a much smaller value.
Fixes: ccf9ef36 ("gallium/u_upload_mgr: eliminate all atomics for the upload buffer")
Closes: #4235 (closed)