drm/amd/display: Skip DIG Check if Link is Virtual for Display Count

[WHY]
Without a check for virtual links, every link's DIG was getting
checked for enabled or disabled. If link was virtual, since it
did not have a DIG, this would cause issues.

[HOW]
Skip DIG Enable check if link is virtual and add virtual link to
to display count.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>