msm: mdss: avoid check for MDP line count during fps update

On latest MDP revisions where there is support for double
buffering of timing generator registers, it is not needed to
check for MDP line count while updating the fps. Add check for
this.

Change-Id: I4b722fb53f856f289e145d0369f1823ff215ab06
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>