msm: mdss: check for fb node refcount before calling null commit

Add a frame buffer reference count check before calling
null commit to release the free list pipes. This avoids
unnecessary flickering on screen if a process tries to open
and close the fb node without performing any operation.

Change-Id: I96b824edccf27f0ddb796c420ef1feb49f65ddea
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>