drm/i915: HSW: allow PCH clock gating for suspend

For the device to enter D3 we should enable PCH clock gating.

v2:
- use HAS_PCH_LPT instead of IS_HASWELL (Ville, Paolo)
- rename lpt_allow_clock_gating to lpt_suspend_hw (Paolo)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>