can we enable RBC only one display connected?

[yugang]

in some devices, most of time, we meet some flickers due to enabling RBC with unknown reason, and also cause the DDB limitation issue. can we disable RBC when more than one display connected. @jennycao @Shao-Feng @js0701 @YuanjunHuang

by the way, some flickers still happen when connecting only one display due to RBC enabled. we still need upstream tracking.

[Shao-Feng]

#603
PR is committed