amd/drm/display: avoid dcn3 on flip opp change for slave pipes

At the moment on flip opp reassignment does not work in all cases
for non root pipes.
This change simply makes sure we prefer pipes not used previously
when splitting in dcn3.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Eric Bernstein <eric.bernstein@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>