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/cmd_pipeline.c b/icd/intel/cmd_pipeline.c
index 4981164..1d6093c 100644
--- a/icd/intel/cmd_pipeline.c
+++ b/icd/intel/cmd_pipeline.c
@@ -1840,9 +1840,9 @@
     }
 
     cmd_wa_gen6_pre_ds_flush(cmd);
-    gen6_3DSTATE_DEPTH_BUFFER(cmd, ds, false);
-    gen6_3DSTATE_STENCIL_BUFFER(cmd, ds, false);
-    gen6_3DSTATE_HIER_DEPTH_BUFFER(cmd, ds, false);
+    gen6_3DSTATE_DEPTH_BUFFER(cmd, ds, fb->optimal_ds);
+    gen6_3DSTATE_STENCIL_BUFFER(cmd, ds, fb->optimal_ds);
+    gen6_3DSTATE_HIER_DEPTH_BUFFER(cmd, ds, fb->optimal_ds);
 
     if (cmd_gen(cmd) >= INTEL_GEN(7))
         gen7_3DSTATE_CLEAR_PARAMS(cmd, 0);