drm/i915/skl: Make the init clock gating function skylake specific

We'll gather cross-gen9 W/A in a separate function later.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>