zink: stop overwriting buffer map pointers for stream uploader

this breaks the driver!

the uploader always maps its own pointer, so modifying that at any
point just explodes things later

Fixes: d179c5d28e ("zink: implement threaded context")

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10787>

src/gallium/drivers/zink/zink_resource.c

@@ -895,7 +895,6 @@ buffer_transfer_map(struct zink_context *ctx, struct zink_resource *res, unsigne
                      (struct pipe_resource **)&trans->staging_res, (void **)&ptr);
          res = zink_resource(trans->staging_res);
          trans->offset = offset;
-         res->obj->map = ptr;
       } else {
          /* At this point, the buffer is always idle (we checked it above). */
          usage |= PIPE_MAP_UNSYNCHRONIZED;