msm: mdss: ignore writeback updates through pan_display path

Updating writeback through pan_display would be pointless as
there is no output buffer available. So, ignore wb updates
through this path.

Change-Id: Icfb0a8252c707eb5ac72810bc3b213aea95b7373
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>