drm/i915: explicitly disable the DIPs we're not using

From this point on, the 'set_infoframe' functions always set the DIP
registers to a known state, so anything done will always be undone at
the modeset.

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