pan/mdg: Add support for multi sample iteration writeout

pan/mdg: Add support for multi sample iteration writeout

Some MSAA+fmt combination require writeout to be split. Right now, it
only impacts blend shaders since we only support MSAA 4x, and the only
formats that could exceed the 128bit/pixel limit in MSAA 4x are
not supported by the fixed-function blend unit. We thus rely on the
blend shader to split things properly. Things will change once we add
MSAA 8x/16x to the mix, since even the blendable formats will exceed
the 128b/pixel limit in that case.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7984>