disp: msm: dp: remove session lock for power state update

Remove session lock for power state update to improve functionality
for fast hotplug use cases. This allows the controller and aux modules
to be updated asynchronously based on hotplug state. For example,
in disconnected state all aux transactions will be aborted.
Updates to the power state can be done without the session lock
since this path is only exercised during compliance testing.

Change-Id: Ibe175624ef56d9f7b953b9b54928682f1cac906a
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>