Revert "drm/i915: Enable RC6 on Ironlake."

This reverts commit ce171780

.

This commit has been independently bisected a few times as being the cause
of a s2ram failure.

Reported-and-tested-by: Kyle McMartin <kyle@mcmartin.ca>
Reported-and-tested-by: Andy Isaacson <adi@hexapodia.org>
Cc: Zou Nan hai <nanhai.zou@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>