iris: Use the BO cache even for coherent buffers on non-LLC.

iris: Use the BO cache even for coherent buffers on non-LLC.

We mark snooped BOs as non-reusable, so we never return them to the
cache.  This means that we'd need to call I915_GEM_SET_CACHING to make
any BO we find in the cache snooped.  But then again, any BO we freshly
allocate from the kernel will also be non-snooped, so it has the same
issue.  There's really no reason to skip the cache - we may as well use
it to avoid the I915_GEM_CREATE overhead.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>