msm: camera: Fix ISP output buffer handling

Clear buffer only if WM's is not configured instead of during
get buffer function. This is needed because sometime we need to
continue use same buffer in case of new buffer is not available.

Change-Id: I33fc9ef1f7cfbffee2e76190165be43d2c637991
Signed-off-by: Konstantin Motov <kmotov@codeaurora.org>