i965: Use the brw_depthbuffer atom on all gens

i965: Use the brw_depthbuffer atom on all gens

The only reason why we had two atoms was that the one we used for gen7+
depended on _NEW_DEPTH and _NEW_STENCIL as well as _NEW_BUFFERS.  Since
this is no longer true, we can combine them into one atom.  We do add a
dependence on BRW_NEW_AUX_STATE but that should never get set on gen4-5
so adding it is a no-op for those platforms.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>