gallium/u_threaded: fix pipe_resource leak for staging transfer

gallium/u_threaded: fix pipe_resource leak for staging transfer

In 2900f82e191 I mistakenly used tc_set_resource_reference in both
tc_transfer_unmap and tc_call_transfer_unmap.

This causes a leak because tc_call_transfer_unmap clears dst before
acquiring a reference, so it must only be used when initializing
tc_payloads.

This fixes the perf drop reported by Marek in MR 7098.

Fixes: 2900f82e191 ("gallium/u_threaded: fix staging and non-staging conflicts")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8035>