drm/amd/display: add way to determine if link is active

[Why]
Need to understand whether link is active aside from stream state.
This could be used to check what links are enabled by GOP.

[How]
Store link_active state in link status and initialize it by checking
if the DIG is enabled.
Keep it updated on every link enable and disable

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>