i965: Only write program to cache when it doesn't exist yet

i965: Only write program to cache when it doesn't exist yet

Current logic re-writes the same data when existing data is found.
Not that this actually matters at the moment in practice, the
contraint for finding matching data is too severe to ever allow
data to be shared between two items in the cache.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>