drm/amd/display: Move stereo timing check to helper

Rework dml2_map_dc_pipes to keep the logic clean.

Reviewed-by: Chaitanya Dhere <chaitanya.dhere@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>