drm/amd/display: check pipe_ctx is split pipe or not

[Why]
Driver use pipe_ctx to reallocate payload may cause allocate
payload twice on same sink with split pipe.

[How]
Drvier must to check pipe_ctx is split pipe or not to avoid
reallocate payload twice on same sink.

Signed-off-by: Paul Hsieh <paul.hsieh@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>