iris: Skip repeated depth buffer disables.

iris: Skip repeated depth buffer disables.

Often times, the depth buffer is entirely disabled, but color render
targets change.  For example, GenerateMipmaps will change the color
render target for each miplevel, but there is no depth buffer.

In the Civilization VI benchmark, this drops the median number of
3DSTATE_DEPTH_BUFFER etc. packets emitted per frame from 472 to 34.