drm/i915/skl: Enable Gen9 RC6

Configure and enable RC6 for Gen9.

v2: Rebase on top of BDW rc6 support (Damien)

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Zhe Wang <zhe1.wang@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>