pan/mdg: Move writeout lowering to common panfrost

These will be used in the Bifrost compiler, albeit for a slightly
different purpose.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7446>
6 files changed