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>
78 jobs for !6980 with panfrost-genxml-part2 in 16 minutes and 48 seconds (queued for 21 seconds)
merge request