[Why]
When IGT's kms_hdmi_inject forces EDID for HDMI audio, dc rejects the
request because virtual signal is not in dc_is_audio_capable_signal
function.

[How]
Includes SIGNAL_TYPE_VIRTUAL as audio capable.
Reviewed-by: Chao-kai Wang <Stylon.Wang@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Wenchieh Chien <wenchieh.chien@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>