video/hdmi: Introduce a generic hdmi_infoframe union

And a way to pack hdmi_infoframe generically.

Cc: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>