msm: mdss: hdmi: return cable status only when HDMI is powered

When audio driver requests the cable status, return that
the cable status is connected only when HDMI is powered.
This prevents audio from accessing HDMI registers when
HDMI clock is off.

Change-Id: Icc5016df731725de25b4ca86e2ac3632ab89d5bf
Signed-off-by: Casey Piper <cpiper@codeaurora.org>