iris: Move depth/stencil flushes so they actually do something

iris: Move depth/stencil flushes so they actually do something

Commit d6dd57d43cd (iris: Add missing depth cache flushes) added the
depth/stencil flushes to the wrong place.  I meant to add them to the
iris_upload_dirty_render_state code that emits the packets, but I
accidentally added them to the nearly identical looking code in
iris_restore_render_saved_bos.  This meant we missed the actual flushing
at draw time, but instead did pointless flushing on the first draw in a
batch where things are already flushed anyway.

This commit moves them to iris_resolve.c, next to the depth prepares,
similar to what we do for color buffers.  i965 does them elsewhere, but
I'm not sure why - this seems like the most consistent place.