etnaviv: Fix rework ZSA into a derived state
In case the stencil is modified, it is also enabled. That was the behavior of the original code, which was also the correct behavior, so reinstate the behavior.
Fixes dEQP-GLES2.functional.fragment_ops.depth_stencil.* on STM32MP1 GC400T.
Fixes: b29fe26d ("etnaviv: rework ZSA into a derived state")