drm/amd/display: Split pipe for stereo timings

[Why & How]
DML2 did not carry over DML1 logic that splits pipe for stero timings. Pipe
splitting is needed in this case to pass stereo tests.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Qingqing Zhuo <Qingqing.Zhuo@amd.com>
Signed-off-by: Taimur Hassan <syed.hassan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>