i965: Fix fast clear of depth buffers.

From section 4.4.7 (Layered Framebuffers) of the GLSL 3.2 spec:

    When the Clear or ClearBuffer* commands are used to clear a
    layered framebuffer attachment, all layers of the attachment are
    cleared.

This patch fixes the fast depth clear path.

Fixes piglit test "spec/!OpenGL 3.2/layered-rendering/clear-depth".

Cc: "10.0" <mesa-stable@lists.freedesktop.org>

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/brw_clear.c

@@ -181,8 +181,16 @@ brw_fast_clear_depth(struct gl_context *ctx)
     */
    intel_batchbuffer_emit_mi_flush(brw);
 
-   intel_hiz_exec(brw, mt, depth_irb->mt_level, depth_irb->mt_layer,
-		  GEN6_HIZ_OP_DEPTH_CLEAR);
+   if (fb->NumLayers > 0) {
+      assert(fb->NumLayers == depth_irb->mt->level[depth_irb->mt_level].depth);
+      for (unsigned layer = 0; layer < fb->NumLayers; layer++) {
+         intel_hiz_exec(brw, mt, depth_irb->mt_level, layer,
+                        GEN6_HIZ_OP_DEPTH_CLEAR);
+      }
+   } else {
+      intel_hiz_exec(brw, mt, depth_irb->mt_level, depth_irb->mt_layer,
+                     GEN6_HIZ_OP_DEPTH_CLEAR);
+   }
 
    if (brw->gen == 6) {
       /* From the Sandy Bridge PRM, volume 2 part 1, page 314: