video/hdmi: Constify infoframe passed to the pack functions

Let's make the infoframe pack functions usable with a const infoframe
structure. This allows us to precompute the infoframe earlier, and still
pack it later when we're no longer allowed to modify the structure.
So now we end up with a _check()+_pack_only() or _pack() functions
depending on whether you want to precompute the infoframes or not.
The names aren't great but I was lazy and didn't want to change all the
drivers.

v2: Deal with exynos churn
    Actually export the new funcs
v3: Fix various documentation fails (Hans)

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: linux-media@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180921143332.28970-1-ville.syrjala@linux.intel.com
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>