drm/i915: Fix check with IS_GEN6

IS_GEN6 missed to include SandyBridge mobile chip, which failed in
i915_probe_agp() for memory config detection. Fix it with a device
info flag.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>