drm/amd/display: Add affected crtcs to atomic state for dsc mst unplug

[Why]
When display topology changed on DSC hub we add all crtcs with dsc support to
atomic state.
Refer to patch:"drm/amd/display: Trigger modesets on MST DSC connectors"
However the original implementation may skip crtc if the topology change
caused by unplug.
That potentially could lead to no-lightup or corruption on DSC hub after
unplug event on one of the connectors.

[How]
Update add_affected_mst_dsc_crtcs() to use old connector state
if new connector state has no crtc (undergoes modeset due to unplug)

Fixes: 44be939f ("drm/amd/display: Trigger modesets on MST DSC connectors")

Reviewed-by: Hersen Wu <hersenwu@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>