drm/i915/cnl: Configure EU slice power gating.

Cannonlake also supports slice power gating on devices with more
than one slice as SKL. Let's assume that this is the same for SKL+
and exclude BXT only.

v2: Also remove KBL.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496781040-20888-7-git-send-email-rodrigo.vivi@intel.com