panfrost: Flesh out pixel kill / zs update

panfrost: Flesh out pixel kill / zs update

Add some notes on what the useful combinations do and flesh out the
logic. Fixes fails in
dEQP-GLES31.functional.image_load_store.early_fragment_tests.*

It's still not entirely clear to me what the difference between "force
early" and "weak early" is from the driver's perspective, though I
suspect there are performance implications to getting it wrong. Follow
what the blob does in case of doubt..

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9105>