video/hdmi: Don't let the user of this API create invalid infoframes

To set the active aspect ratio value in the AVI infoframe today, you not
only have to set the active_aspect field, but also the active_info_valid
bit. Out of the 1 user of this API, we had 100% misuse, forgetting the
_valid bit. This was fixed in:

  Author: Damien Lespiau <damien.lespiau@intel.com>
  Date:   Tue Aug 6 20:32:17 2013 +0100

      drm: Don't generate invalid AVI infoframes for CEA modes

We can do better and derive the _valid bit from the user wanting to set
the active aspect ratio.

v2: Fix multi-lines comment style (Thierry Reding)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>