drm/i915/skl: Provide a placeholder for init_clock_gating()

v2: Rebase on top of the broadwell_init_clock_gating() name change

Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>