iris: use 0 for TCS passthrough program string ID

iris: use 0 for TCS passthrough program string ID

the passthrough shader doesn't need a real program string ID - that's
basically used for ARB programs indicating total program source code
changes, or other pre-baked uniform changes, etc...none of which a
passthrough shader has...so we don't need a unique identifier to
distinguish them.  We want to use a consistent value so we find
existing passthrough shaders in the cache.