drm/sun4i: hdmi: Switch to container_of_const

container_of_const() allows to preserve the pointer constness and is
thus more flexible than inline functions.

Let's switch all our instances of container_of() to container_of_const().

Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222-kms-hdmi-connector-state-v7-34-8f4af575fce2@kernel.org