drm/amd/display: program manual trigger only for bottom most pipe

[Why]
We only want to manual trigger end-of-frame when the bottom-most
pipe is flipped to prevent overlays from ending the frame too early.

[How]
Check that the manual trigger is only firing on bottom plane.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>