drm/i915/skl: Double RC6 WRL always on

WaRsDoubleRc6WrlWithCoarsePowerGating should
be enabled for all Skylakes. Make it so.

Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449505785-20812-2-git-send-email-mika.kuoppala@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>