lavapipe: fix pipeline handling for dynamic render zs state with pipeline library

lavapipe: fix pipeline handling for dynamic render zs state with pipeline library

this is slightly more nuanced: if this is ONLY a fragment state,
pDepthStencilState must exist, but if it's BOTH fragment and output,
then pDepthStencilState must only exist if zs attachment is used

Fixes: d4d5a7abba7 ("lavapipe: implement EXT_graphics_pipeline_library")

Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16457>