msm: mdss: fix handling of audio for DVI sinks

Certain sinks (such as DVI monitors) do not support audio
which can be identified using the sink's EDID. For such sinks,
ensure that audio is not routed over the external display.

Change-Id: I1c1a634bf096392094b5fd868a87385764657e6b
CRs-Fixed: 1093442
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>