i965: Add shader cache support for vertex and fragment stages

i965: Add shader cache support for vertex and fragment stages

This enables the cache on vertex and fragment shaders only.

v2:
 * Use MAYBE_UNUSED. (Matt)

[jordan.l.justen@intel.com: reword subject]
[jordan.l.justen@intel.com: *_cached_program => brw_disk_cache_*_program]
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>