drm/vc4: hdmi: Check the device state in prepare()

Even though we already check that the encoder->crtc pointer is there
during in startup(), which is part of the open() path in ASoC, nothing
guarantees that our encoder state won't change between the time when we
open the device and the time we prepare it.

Move the sanity checks we do in startup() to a helper and call it from
prepare().

Fixes: 91e99e11 ("drm/vc4: hdmi: Register HDMI codec")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>