panfrost: Clean up blend shader errata handling

panfrost: Clean up blend shader errata handling

The cases of SFBD blend shaders (which work normally) and MFBD blend
shaders (which need a bizarre errata workaround) are distinct. Let's
seperate them to make each a bit clearer.

Since this field is repurposed on Bifrost, this should fix bugs there
too (although blend shaders on Bifrost are currently todo).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6440>