i965: Fix fast clear of depth buffers.

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>