i965/clear: Don't perform redundant depth clears

i965/clear: Don't perform redundant depth clears

We already have this little optimization for color clears.  Now that
we're actually tracking whether or not a slice has any fast-clear
blocks, it's easy enough to add for depth clears too.

Improves performance of GFXBench 4 TRex at 1920x1080 by:
- Skylake GT4: 0.905932% +/- 0.0620197% (n = 30)
- Apollolake:  0.382434% +/- 0.1134730% (n = 25)

v2: (by Ken) Rebase and drop intel_mipmap_tree.c changes, as they're
    no longer necessary (other patches already landed to do that part)

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