mdss: Dynamic timeout calculation for cmd stop

Time out condition was hard coded to 60 fps. If dynamic fps
feature was used, an invalid timeout might occur.

Change-Id: I0abe52416c10ef4ae0af6f541d22c66f6c8a6b5e
Signed-off-by: Terence Hampson <thampson@codeaurora.org>
Signed-off-by: MassStash <MassStash@gmail.com>