drm/msm/dpu: fix comparing pointer to 0 in dpu_vbif.c

Fixes coccicheck warning:

drivers/gpu/drm/msm/disp/dpu1/dpu_vbif.c:27:51-52: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_vbif.c:109:51-52: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_vbif.c:167:12-13: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_vbif.c:167:22-23: WARNING comparing pointer to 0

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>