drm/i915: fix TV mode setting in property change

Only set TV DAC in property change seems doesn't work, we have to
setup whole crtc pipe which assigned to TV alone.

Signed-off-by: Zhenyu Wang <zhenyu.z.wang@intel.com>
[anholt: Note that this should also fix the oops at startup with new 2D]
Signed-off-by: Eric Anholt <eric@anholt.net>