drm/amd/display: Fix writeback_info is not removed

[WHY]
Counter j was not updated to present the num of writeback_info when
writeback pipes are removed.

[HOW]
update j (num of writeback info) under the correct condition.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>