i965/gen4: Remove redundant check for depth when rebasing stencil

i965/gen4: Remove redundant check for depth when rebasing stencil

In case of gen < 6 stencil (if present) is always combined with
depth. Both stencil and depth attachments point to the same
physical surface.
Alignment workaround starts by considering depth and updates
stencil accordingly. Current logic continues with stencil and
in vain considers the case where depth would refer to different
surface than stencil.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>