drm/i915: move clock gating functionality into intel_pm module

This moves the clock gating-related functions into intel_pm module.

Also, please note that we do change the function type from static to
non-static in this patch for the move, to prevent breaking bisecting with
non-working intermediate commit. Those are returned back to static form in
the following patch which setups a generic PM initialization function,
which was split into a different one to simplify review.

v2: rebase on top of latest drm-intel-next-queued to incorporate all the
changes that went there meanwhile.

Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>