intel: enable HiZ on Gen7+

The support is restricted to Gen7+.  To support HiZ on Gen6, we need to
perform manual combined/separated stencil resolves and that may hurt the
performance instead.

Also, we do not support fast depth clears.  xglSetFastClearDepth() will be
removed in a later xgl.h revision.
diff --git a/icd/intel/fb.h b/icd/intel/fb.h
index b1526c7..f7c6b1f 100644
--- a/icd/intel/fb.h
+++ b/icd/intel/fb.h
@@ -36,6 +36,7 @@
     uint32_t rt_count;
 
     const struct intel_ds_view *ds;
+    bool optimal_ds;
 
     uint32_t sample_count;
     uint32_t width;