gallium/u_threaded: don't reference resource in pipe_transfer

gallium/u_threaded: don't reference resource in pipe_transfer

The atomic can be costly, so skip it. This should be safe because
we never have pipe_transfer as the only live reference of a buffer.

Drivers can't do this yet because something needs to hold the resource
reference while a transfer object is being passed via u_threaded_context.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10662>