drm/msm/dp: add private state to dp_mst_bridge

Current dp_mst_bridge has variables that are accessed by both
check phase and commit phase, which causes racing issues. This
change will add private state to dp_mst_bridge to separate check
phase and commit phase.

Change-Id: I8ac05cf5f1755375e4e9f34e42dbaea1d23bac64
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>