gallium/u_transfer_helper: clean up boolean flags

gallium/u_transfer_helper: clean up boolean flags

Whenever we add or remove a flag here, we need to update a bunch of
drivers in a fragile way. Moving to flags here instead should make this
a bit easier to maintain in the future.

Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17959>