drm/amd/display: Handle virtual signal type in disable_link()

[Why]
The new implementation changed the behavior to allow process setMode
to DAL when DAL returns empty mode query for unplugged display.
This will trigger additional disable_link().
When unplug HDMI from MST dock, driver will update stream->signal to
"Virtual". disable_link() will call disable_output() if the signal type
is not DP and induce other displays on MST dock show black screen.

[How]
Don't need to process disable_output() if the signal type is virtual.

Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>