Only disable RC6 on Sandybridge GT1 and not on all intel hardware

This reworks the RC6 disable policy. For now only users with Sandybridge GT1 gpus got into severe trouble when running with rc6 enabled (black screen, freeze). This small kernel patches automatically identifies Sandybridge GT1 gpus and disables RC6 for them.

If user still wants to manually enable (no idea why) the normal i915.rc6 params stay working.