i965: add cache fallback support using serialized nir

i965: add cache fallback support using serialized nir

If the i965 gen program cannot be loaded from the cache, then we
fallback to using a serialized nir program.

This is based on "i965: add cache fallback support" by Timothy Arceri
<timothy.arceri@collabora.com>. Tim's version was written to fallback
to compiling from source, and therefore had to be much more complex.
After Connor and Jason implemented nir serialization, I was able to
rewrite and greatly simplify this patch.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>