drm/vc4: hdmi: Create destroy state implementation

Even though we were rolling our own custom state for the vc4 HDMI
controller driver, we were still using the generic helper to destroy
that state.

It was mostly working since the underlying state is the first member of
our state so the pointers are probably equal in all relevant cases, but
it's still fragile so let's fix this properly.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231207-kms-hdmi-connector-state-v5-18-6538e19d634d@kernel.org