i915: Add an option for testing the effect of early Z in classic mode.

The early Z stuff is supposed to be unsafe without some more work in the
enable/disable path (in particular, how do we want to get it disabled on
the way out to the X Server?), but at the moment is 6% in OA.

src/mesa/drivers/dri/intel/intel_context.c

@@ -729,6 +729,7 @@ intelInitContext(struct intel_context *intel,
    }
    intel->use_texture_tiling = driQueryOptionb(&intel->optionCache,
 					       "texture_tiling");
+   intel->use_early_z = driQueryOptionb(&intel->optionCache, "early_z");
 
    intel->prim.primitive = ~0;