i965: Shut up Coverity warning about HiZ buffers.

i965: Shut up Coverity warning about HiZ buffers.

Here the AUX_USAGE_* mode indicates that we have HiZ, so we will have
a HiZ buffer.  But Coverity doesn't know that, so it thinks it might
be NULL because we checked hiz_buf != NULL earlier.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>