drm/i915: implement WADP0ClockGatingDisable

Found in Bspec vol4h South Display Engine Registers [CPT, PPT],
section "5.3.1  TRANS_CHICKEN_1—Transcoder Chicken Bits 1"

v2: Make it compile.

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