panfrost: Fix fixed-function blend on Mali v6
Mali v6 (G72) doesn't support constants in blend equations, let's use a shader in that case. Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com> Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com> Part-of: <!6980>