drm/vc4: hdmi: Deal with multiple debugfs files

The HDMI driver was registering a single debugfs file so far with the name
hdmi_regs.

Obviously, this is not going to work anymore when will have multiple HDMI
controllers since we will end up trying to register two files with the same
name.

Let's use the ID to avoid that name conflict.

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