drm/msm/sde: add proper mode config lock to plane QoS check

Realtime QoS status is determined in plane atomic check using
mode config for_each macro.  This macro requires locking of
mode config lock.  Add lock to connector enumeration to avoid
any race conditions.

Change-Id: I6142536268c9261a7af12a8615fa0a2de17ba8e8
Signed-off-by: Alan Kwong <akwong@codeaurora.org>