drm/amd/display: fix missing writeback disablement if plane is removed

[Why]
If the plane has been removed, the writeback disablement logic
doesn't run

[How]
fix the logic order

Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Roy Chan <roy.chan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>