drm/vc4: hdmi: Fix register offset with longer CEC messages

The code prior to 311e305f ("drm/vc4: hdmi: Implement a register
layout abstraction") was relying on the fact that the register offset
was incremented by 4 for each readl call. That worked since the register
width is 4 bytes.

However, since that commit the HDMI_READ macro is now taking an enum,
and the offset doesn't increment by 4 but 1 now. Divide the index by 4
to fix this.

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>