drm/amd/display: Disable force_single_disp_pipe_split on DCN2+

[WHY]
force_single_disp_pipe_split is a debug flag for use on DCN1
but isn't necessary otherwise as DCN2+ splits by default

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