drm/i915: Call intel_{dp, hdmi}_prepare for chv

CHV was forgotten the intel_{dp,hdmi}_prepare() were introduced (or the
chv patches were still in flight?). Call these when enabling the ports.

Things tend to work much better when we actually write something
to the port registers :)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>