panfrost: Do per-sample shading when outputs are read

Fixes dEQP-GLES31.functional.blend_equation_advanced.msaa.*

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5930>
1 file changed