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>