drm/amd/display: Check link_active instead of lane_settings != unknown

[Why]
enable_link_dp_mst checks that cur_link_settings != unknown to determine
that the link is already enabled, to skip redundant enablement calls for
multiple streams on the same link. During dc_reinitialize_hardware,
cur_link_settings on previously-active links is not cleared, which blocks
MST links from being re-enabled after a reinitialization.

[How]
 - check for link_status->link_active instead, as it's the real intent
 - clear cur_link_settings when we clear link_active

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>