drm/i915/bxt: add bxt_init_clock_gating

v2:
- Make the condition to select between SKL and BXT consistent with the
  corresponding condition in init_workarounds_ring (Nick)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>