turnip: Enable LRZ testing (not writing) in the presence of discards.

We can LRZ test to not rasterize maybe-discarded pixels, as long as we don't LRZ write a Z that may be discarded. We can drop this check, because LRZ writes are already disabled by TU_LRZ_FORCE_DISABLE_WRITE, and tu_6_build_depth_plane_z_mode() uses has_kill to choose EARLY_LRZ_LATE_Z (or just LATE_Z if LRZ isn't enabled).

ANGLE perf:

trex_200          +11.7208% +/- 0.476166% (n=3)
aztec_ruins_high  +11.9138% +/- 0.147586% (n=3)