drm/i915/skl: Gen9 coarse power gating

Enable coarse power gating for Gen9. This feature allows render and
media engine to enter RC6 independently. Policies are configured
together with RC6. This feature will only be enabled when RC6 is
enabled.

v2: Rebase after Chris'/Mika's forcewake change (Damien)

Reviewed-by: Damien Lespiau <damien.lespiau@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>