drm/amd/display: skip audio config for virtual signal

[WHY & HOW]
IGT's kms_hdmi_inject (subtest inject_audio) triggers a warning message
from dce_aud_az_configure. This can be fixed by checking Virtual signal
that does not need to configure AZ audio since it does not have any.

Reviewed-by: Rodrigo Siqueira <[e-mail]>
Acked-by: Wayne Lin <[e-mail]>
Signed-off-by: Alex Hung <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/dce/dce_audio.c

···
 	bool is_ac3_supported = false;
 	union audio_sample_rates sample_rate;
 	uint32_t strlen = 0;
+
+	if (signal == SIGNAL_TYPE_VIRTUAL)
+		return;
+
 	value = AZ_REG_READ(AZALIA_F0_CODEC_PIN_CONTROL_HOT_PLUG_CONTROL);
 	set_reg_field_value(value, 1,
 			AZALIA_F0_CODEC_PIN_CONTROL_HOT_PLUG_CONTROL,