zink: rework oom flushing

zink: rework oom flushing

tracking mem usage on the context is bad because if an app is always using
a large amount of memory across multiple batches, it triggers the oom flushes
on every flush and forces fencing

instead, add an explicit flag for oom flushing and another for stalling, then
flag oom flushing and stalling as needed, with set_framebuffer_state being an
additional flush point now since it's guaranteed not to split renderpasses

also proactively prune pending batch states if there's a lot of them

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11965>