drm/i915: bdw: fix RC6 enabled status and disable runtime PM

On BDW we don't enable RC6 at the moment, but this isn't reflected in
the (sanitized) i915.enable_rc6 option. So make enable_rc6 report
correctly that RC6 is disabled, which will also effectively disable RPM
on BDW (since RPM depends on RC6).

Signed-off-by: Imre Deak <imre.deak@intel.com>