drm/amd/display: fix dcn20 global sync dml param extraction

Currently the paremeters are extracted as if dml is calculating
using pipes as we pass them in. in reality, dml internally merges
pipes into planes if pipe split is detected.

This change adds reverse logic to dcn20_calculate_dlg_params so
that the global sync parameters can be correctly extracted for
all the pipes when pipe split is enabled.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>