drm/amd/display: Adjust reg field for DSC wait for disconnect

drm/amd/display: Adjust reg field for DSC wait for disconnect

[WHY]
DSC was waiting for the wrong field to disconnect cleanly.

[HOW]
Changed field the DSC disconnect was waiting on.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ryan Seto <ryanseto@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>