drm/msm/sde: reset both Layer mixers during Writeback 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: I7ff9dd0dc077108ae5a3e0d98f65fba7a55600dc
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>