msm_fb: display: Add null check for base layer

Add null check for base layer in mdp4_overlay_borderfill_stage_up
to avoid NULL pointer dereference, and kernel panic.

Change-Id: Icc6db4807274bd44aafdabc3d8b8dae70587d8d0
Signed-off-by: Neti Ravi Kumar <ravineti@codeaurora.org>
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>