Previously, we just hashed the entire descriptor set layout verbatim.
This meant that a bunch of extra stuff such as pointers and reference
counts made its way into the cache.  It also meant that we weren't
properly hashing in the Y'CbCr conversion information information from
bound immutable samplers.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>