i965/gen6 depth surface: calculate depth (array size) for depth surface

i965/gen6 depth surface: calculate depth (array size) for depth surface

(bc1acaa for gen6)

This will be used in 3DSTATE_DEPTH_BUFFER in a later patch.

Note: Cube maps are treated as 2D arrays with 6 times as
many array elements as the cube map array would have.

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>