drm/vc4: A present but empty dmas disables audio

Overlays are unable to remove properties in the base DTB, but they
can overwrite them. Allow a present but empty 'dmas' property
to also disable the HDMI audio interface.

See: https://github.com/raspberrypi/linux/issues/2489



Signed-off-by: Phil Elwell <phil@raspberrypi.org>