drm/amd/display: Add a check-function for virtual signal type

[why]
Same functions exist for all other signal types.

[how]
Add a function that checks against virtual signal type.
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -102,4 +102,9 @@ static inline bool dc_is_audio_capable_signal(enum signal_type signal)
 		dc_is_hdmi_signal(signal));
 }
 
+static inline bool dc_is_virtual_signal(enum signal_type signal)
+{
+	return (signal == SIGNAL_TYPE_VIRTUAL);
+}
+
 #endif