msm: mdss: fix youtube video playback issue

Add check in smart blit condition to avoid smart blit if
MDP_IS_FG is not set for background layer. This is needed to preserve
FB content across multiple blit request lists.

Change-Id: I9c2440d241fdc296ea85e795c251af84d011cab3
Signed-off-by: Ramakant Singh <ramaka@codeaurora.org>