subsurface: Fix subsurface stack order error

subsurface: Fix subsurface stack order error

Let's say there are two windows. One consists of a main surface and two
sub-surfaces placing above and below of the main surface each. The other
one consists of a main surface and a sub-surface placing above of the
main surface. Let's call these windows A and B in order.

The problem is that if the main surface of A window is restacked above
of the main surface of B window, the A window ended up placing between
the B's sub-surface and the main surface of B. This patch is to fix this
error.

Change-Id: Id06d833b75ca184e4eb55fb024fbf8e9356f8620