freedreno/a6xx: more early-z

freedreno/a6xx: more early-z

Technically we only have to do late-z in the alpha-test or discard case
if depth-write is enabled.  If depth write is disabled, the depth read /
test / conditional-write interlock that we need to emulate is not a
problem, so we can still use early-z test.

There is a slightly weird case when there is no zsbuf attachment (see
dEQP-GLES31.functional.fbo.no_attachments.*) where the hw wants us to
use LATE_Z.. not entirely sure if this is an interaction with occlusion
query or just a pecularity of how the hw works when there is no depth
buffer.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5336>