st/mesa/i965: create link status enum

st/mesa/i965: create link status enum

For the on-disk shader cache we want to be able to differentiate
between a program that was linked and one that was loaded from cache.

V2:
 - don't return the new enum directly to the application when queried,
   instead return GL_TRUE or GL_FALSE as required. Fixes google-chrome
   corruptions when using cache.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>