msm: mdss: properly handle panel on and off

There is some condition that panel off will be called multiple times.
In this case, it was blindly free the gpios that were already released.
This is adding the check to handle this condition.

Change-Id: I0140872be1255fe00ed5abd164a246fa57b2a245
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>