drm/msm/sde: stage border fill on both layermixers in WB disable

During writeback path disable in 2 Layer mixer case, both
of them needs to be reset by staging border fill. But only
one layer mixer is reset completely. Change is made to reset
both in write back disable case.

Change-Id: I5255b72caa5610a89e804a36398750acf667ff32
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>