drm/sti: allow audio playback on HDMI even if disabled.

This fix allows to play audio while HDMI is disconnected.
When HDMI is disable, audio configuration is stored and samples
are dropped (by HDMI IP).
When HDMI is enabled, audio HDMI configuration is applied and samples
are outputted on HDMI wire.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>