gallium/u_upload_mgr: lower risk of hitting an assert

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.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Fixes: ccf9ef36285 ("gallium/u_upload_mgr: eliminate all atomics for the upload buffer")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4235
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8880>