d3d12: do not save away the passthrough-shader in the blitter-state
If we store away a passthrough shader here, then we'll end up deleting when unbinding the GS for the blitter, and then trying to re-bind it again.
This is just a band-aid; we should probably not bind the passthrough shader in a way that makes it appear to be bound in the first place intead. But that's a bigger change.
This fixes: spec@ext_framebuffer_multisample@point-smooth 16