drm/i915/skl: Gen9 Forcewake

Implement common forcewake functions shared by Gen9 features.

v2: Make the focewake_{get,put} functions static (Mika)
    Small coding style fix in the function definition (Damien)

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Zhe Wang <zhe1.wang@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v2)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>