drm/i915: mask the video DIP frequency when changing it

Better safe than sorry. Currently we never change the frequency and
use the same for every infoframe type, so the only way to reproduce a
bug would be with the BIOS doing something.

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>