pan/mdg: Dual source blend input/writeout support

We write to r2, which is preseved through to the blend shader, from
where it is read. We won't worry about MRT to keep things simple.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5620>
9 files changed