i965: Change signature of brw_get_depthstencil_tile_masks()

i965: Change signature of brw_get_depthstencil_tile_masks()

Add new parameters `depth_level` and `depth_layer`, which specify depth
miptree's slice of interest.  A following patch will pass the new
parameters through to intel_miptree_slice_has_hiz().

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>