i965: Allow disk shader cache usage with LINKING_SUCCESS status

i965: Allow disk shader cache usage with LINKING_SUCCESS status

Currently, we only look in the disk shader cache if we see that the
shader program is in the cache during the link step.

If the shader cache entry isn't found during the program link, there
are still some (fairly unlikely) scenarios where later it might be
useful to search the cache for gen binary programs.

1. If the cache evicts the serialized glsl cache, there might still be
   valid gen program entries in the disk cache.

2. If two applications are running in parallel, then it is possible
   that one may write out the cached gen program item which the other
   application can then make use of.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>