etnaviv: switch depth mode when linear PE is used

Early depth test is broken when the linear render target mode is used and depth is written from the PE stage. It seems RA and PE disagree about the cache layout in that case, so the RA sees unwritten/invalid depth cache lines leading to random depth test fails. Early test works fine if depth is written from the RA stage.

To work around this issue, detect the combination of linear RT, early test and late write and switch to late test in that case.