i965/miptree: Zero-initialize CCS_D buffers

Before this patch, the aux_state was actually AUX_INVALID because the BO
was never defined. This was fine on single slice miptrees because we
would fast-clear the resource right after creation. For multi-slice
miptrees on SKL+ however, this results in undefined behavior when
accessing a non-base slice. Here's a specific example:

1) Fast clear level 0
   * Undefined CCS_D buffer allocated in "PASS_THROUGH" state.
   * Level 0 transitions to the CLEAR state.
2) Render to level 1
   * Level 1 may have a 2-bit pattern of 2's.
   * Rendering with a 2 in the CCS is undefined.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 8a949105)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/mesa/drivers/dri/i965/intel_mipmap_tree.c