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.c b/icd/intel/fb.c
index e2115af..f6c0bad 100644
--- a/icd/intel/fb.c
+++ b/icd/intel/fb.c
@@ -97,8 +97,19 @@
         }
 
         fb->ds = ds;
+
+        switch (att->layout) {
+        case XGL_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL:
+        case XGL_IMAGE_LAYOUT_DEPTH_STENCIL_READ_ONLY_OPTIMAL:
+            fb->optimal_ds = true;
+            break;
+        default:
+            fb->optimal_ds = false;
+            break;
+        }
     } else {
         fb->ds = NULL;
+        fb->optimal_ds = false;
     }
 
     fb->width = width;