drm/amdgpu/display/mst: adjust the logic in 2nd phase of updating payload

[why & how]
adjust the coding in dm_helpers_dp_mst_send_payload_allocation()
for reading easily.

Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>