iris: Only store the SHA1 of the NIR in iris_uncompiled_shader

iris: Only store the SHA1 of the NIR in iris_uncompiled_shader

Jason pointed out that we don't need to keep an entire copy of the
serialized NIR around, we just need the SHA1.  This does change our
disk cache key to be taking a SHA1 of a SHA1, which is a bit odd,
but should work out and be faster and use less memory.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>