i965/gen6 depth surface: calculate minimum array element being rendered

i965/gen6 depth surface: calculate minimum array element being rendered

(a23cfb8 for gen6)

In layered rendering this will be 0. Otherwise it will be the
selected slice.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>