drm/i915: Make AGP=n work even on gen3

Most platforms din't hit this condition, but if we want to allow
building without agp we should also make this allowed on gen3.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>