drm/amd/display: fix and simplify pipe split logic

Current odm/mpc combine logic to detect which pipes need to split
logically is flawed leading to incorrect pipe merge/split operations
being taken.

This change cleans up the logic and fixes the logical errors.

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