panfrost: Add the MALI_WRITES_{Z,S} flags

panfrost: Add the MALI_WRITES_{Z,S} flags

We discovered 2 new shader flags used when a fragment shader updates
the depth/stencil value through a ZS writeout. If those flags are not
set, the depth/stencil value stored in the depth/stencil tilebuffer
remain unchanged.

While at it, rename unknown2 into flags_hi and rename flags into
flags_lo.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3697>