drm/vc4: hdmi: Make sure the device is powered with CEC

Similarly to what we encountered with the detect hook with DRM, nothing
actually prevents any of the CEC callback from being run while the HDMI
output is disabled.

However, this is an issue since any register access to the controller
when it's powered down will result in a silent hang.

Let's make sure we run the runtime_pm hooks when the CEC adapter is
opened and closed by the userspace to avoid that issue.

Fixes: 15b4511a ("drm/vc4: add HDMI CEC support")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>