drm/i915: introduce haswell_init_clock_gating

This is based on Ivy Bridge clock gating for now, but is subject to
changes in the future.

Note: Compared to the ivb clock gating this drops the the IDICOS
medium uncore sharing tuned in

commit 20848223
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri May 4 18:58:59 2012 -0700

    drm/i915: set IDICOS to medium uncore resources

Eugeni wants to benchmark the effect of this first.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
[danvet: added note]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>