drm/i915: disable the infoframe before changing it

That's what the VIDEO_DIP_CTL documentation says we need to do. Except
when it's the AVI InfoFrame and we're ironlake_write_infoframe.

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