msm: mdss: overlay: skip kickoff for empty pan_display during handoff

If an empty pan_display is called before handoff is completed,
the NULL commit would introduce a blank frame. Skip the kickoff,
to avoid displaying blank frame.

CRs-Fixed: 1015574
Change-Id: I7f0af10acaf6d3b1e72e74320d03a2a926ccdd06
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>