drm/i915: Support not having an init clock gating function defined

When enabling new platforms, we may not have any W/A to apply,
especially that, now, a bunch of them have to be done from the ring.

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