drm/amd/display: allow edp updates for virtual signal

[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 <[e-mail]>
Acked-by: Qingqing Zhuo <[e-mail]>
Signed-off-by: Alex Hung <[e-mail]>
Signed-off-by: Wenchieh Chien <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/include/signal_types.h

···
 {
 	return (signal == SIGNAL_TYPE_DISPLAY_PORT ||
 		signal == SIGNAL_TYPE_DISPLAY_PORT_MST ||
+		signal == SIGNAL_TYPE_VIRTUAL ||
 		dc_is_hdmi_signal(signal));
 }