video/hdmi: Allow "empty" HDMI infoframes

video/hdmi: Allow "empty" HDMI infoframes

HDMI 2.0 Appendix F suggest that we should keep sending the infoframe
when switching from 3D to 2D mode, even if the infoframe isn't strictly
necessary (ie. not needed to transmit the VIC or stereo information).
This is a workaround against some sinks that fail to realize that they
should switch from 3D to 2D mode when the source stop transmitting
the infoframe.

v2: Handle unpack() as well
    Pull the length calculation into a helper

Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: linux-media@vger.kernel.org
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113170427.4150-2-ville.syrjala@linux.intel.com
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>