msm: mdss: Fix potential dereferencing of null pointer

During atomic commit on a writeback panel, there is a possibility
of deferencing a NULL pointer if the configuration changes before
the commit. This change adds a NULL pointer check to avoid it.

Change-Id: I56d0efad40992b6f87c81e5eab93cf0f24f6f524
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>