drm/vc4: hdmi: Fix up CEC registers

The commit 311e305f ("drm/vc4: hdmi: Implement a register layout
abstraction") forgot one CEC register, and made a copy and paste mistake
for another one. Fix those mistakes.

Fixes: 311e305f ("drm/vc4: hdmi: Implement a register layout abstraction")
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>