drm/i915: fix unbalanced GT powersave enable / disable calls

Atm, we call intel_gt_powersave_enable() for GEN6 and GEN7 but disable
it for everything starting from GEN6. This is a problem in case of BDW.
Since I don't have a BDW to test if RC6 works properly, just keep it
disabled for now and fix only the disable function.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>