drm/amd/display: Fix FreeSync when RGB MPO in use

[WHY]
We should skip programming manual trigger on non-primary planes when MPO is
enabled.

[HOW]
Implement an explicit mechanism for skipping manual trigger programming
for planes that shouldn't cause the frame to end.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>