msm: mdss: Ensure remove vsync handler called during ctl stop

With current implementation, vsync handler is never removed
in case of shell stop if the call doesn't come from HAL. Due
to this in certain cases the vsync ref count is unbalanced.
MDP driver should ensure that the vsync handler is always
removed explicitly during shell stop. Fix this issue by using
separate variable for SLAVE ctx and using MASTER ctx for
removing vsync handler.

Change-Id: I281b9db0623f9760efea5b2ceb978e648083f156
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>