iris: avoid use after free in shader destruction

iris: avoid use after free in shader destruction

While playing with compute shaders, I was getting a random crash,
noticed that bind_state was using the old shader info for comparision,
but gallium allows the shader to be deleted while bound, so this could
lead to a use after free.

This can't happen using the cso cache. As it tracks all of this.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>