Flag all supported drivers from generation 6 and above as initialized via agp

[bapt]

Summary

In order to simplify porting the native initialization method for intel drivers
from generation 6 and above without breaking currently working and supported
ones, add a init_via_agp flag to keep those on the old method.
Note that when we will remove this flag the fake agp code corresponding in the
agp driver would have to be removed

[dumbbell]

This patch is no longer relevant: the i915 update, committed in rS296548, doesn't use AGP to initialize GEN6+ anymore.