msm: mdss: null check for ctl pointer

display_commit may be called before overlay_on, which passes a null
ctl pointer. Adding a null check to avoid the crash.

Change-Id: Id151c0a90b54044fd9c8a2393489d23db8685c41
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>