drm/i915: implement WADPOClockGatingDisable for LPT

This should prevent mode set failures on LPT.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Pimp the w/a tag to fit into Damien's new scheme.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>