msm: mdss: fix race condition in idle power collapse logic

MDP3 GDSC can be turned off, or power-collased for command mode panels
in static screen use-cases. In the current implementation, the power
collapse is triggered by a worker thread in the interface driver
whenever it enters the idle state.

Use mutex to protect the idle_pc access and to avoid race condition.

Change-Id: I17a824c88e374aee2404da7f57d5ba5f39e314bc
Signed-off-by: Sachin Bhayare <sachin.bhayare@codeaurora.org>