tilcdc: Allow non-audio modes when we don't support them.

Allow non-audio modes on an audio capable monitor if we explicitly
disable audio.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
bbb: Add a fall-back non-audio HDMI cape

When a user cape hogs the audio, allow booting with a HDMI cape which
only supports video.