wined3d: Don't load the draw_binding location in surface_unmap() unless the surface is on a swapchain.

It really makes no sense to do this just because a surface happens to be the
current render target. In particular, this patch prevents needlessly bouncing
the surface between the CPU and the GPU when the application does e.g.
multiple colorkeyed blits to the current render target to draw a HUD.

It's no longer necessary because target paths are always normalized.