i965: Use has_surface_tile_offset in depth/stencil alignment workaround.

Currently, has_surface_tile_offset is equivalent to gen == 4 && !is_g4x.

We already use it for related checks in brw_wm_surface_state.c, so it
makes sense to use it here too.  It's simpler and more future-proof.

Broadwell also lacks surface tile offsets.  With this patch, I won't
need to update any generation checking; I can simply not set the flag.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>