blorp: Stop whacking Z24 depth to BGRA8

blorp: Stop whacking Z24 depth to BGRA8

The shader code required to do this is int(sat(x) * UINT24_MAX) which
isn't really worth all the effort to avoid.  Doing the format
conversion, on the other hand, prevents us from sampling with HiZ which
is something that we very much want on gen8-9 where we can.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>