drm/vc4: hdmi: Clear unused infoframe packet RAM registers

Using a hdmi analyser the bytes in packet ram
registers beyond the length were visible in the
infoframes and it flagged the checksum as invalid.

Zeroing unused words of packet RAM avoids this

Fixes: 21317b3f

 ("drm/vc4: Set up the AVI and SPD infoframes.")
Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Link: https://lore.kernel.org/r/20220613144800.326124-20-maxime@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>