drm/amd/display: Free phantom plane and stream properly

[Description]
Refcount is incremented on allocation and
when adding to the context. Therefore we must
release the phantom plane and stream after
removing from the context.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>