msm: mdss: fix race condition between dfps update and stop

Dynamic fps change and video interface stop works in two
different thread. It is possible that video interface move
to stop state when dynamic fps thread is in process of
updating the fps information on the same interface.

This change acquires the stop mutex lock and releases it
at the end.

Change-Id: I94d279ea53acb7daa7b042829c0e31df758a6782
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>