zink: rework rast-discard for primgen queries

zink: rework rast-discard for primgen queries

I originally used CWE for this case since it nicely avoids a lot of the
"complex" operations while still disabling most effects of a fragment shader

in theory, however, there are some cases where CWE isn't quite the same as actually
disabling the fs, so this adds a new rast-discard mode that can be used for
those cases

it can also be used instead of the dummy surface handling that was previously
used for drivers/hw that didn't support primitivesGeneratedQueryWithRasterizerDiscard,
which enables a bunch of gross conditionals to be simplified

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24676>