drm/i915: Disable RC6 on Sandybridge by default

RC6 fails again.

> I found my system freeze mostly during starting up X and KDE. Sometimes it
> works for some minutes, sometimes it freezes immediatly. When the freeze
> happens, everything is dead (even the reset button does not work, I need to
> power cycle).

> I disabled RC6, and my system runs wonderfully.

> The system is a Z68 Pro board with Sandybridge i5-2500K processor, 8
> GB of RAM and UEFI firmware.

Reported-by: Kai Krakow <hurikhan77@gmail.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>