drm/amd/display: Fix MPO & pipe split on 3-pipe dcn2x

[WHY]
DML is incorrectly initialized with 4 pipes on 3 pipe configs
RequiredDPPCLK is halved on unsplit pipe due to an incorrectly handled 3 pipe
case, causing underflow with 2 planes & pipe split (MPO, 8K + 2nd display)

[HOW]
Set correct number of DPP/OTGs for dml init to generate correct DPP topology
Double RequiredDPPCLK after clock is halved for pipe split
and find_secondary_pipe fails to fix underflow

Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>